The problem with swapping ammonia for water is that unlike water, ammonia ice is denser than liquid ammonia and therefore sinks instead of floating as ice does in water.
The layer of ice that forms on water isolates the body of water underneath preventing it from freezing further but with ammonia, the top freezes, sinks, exposes the next layer which freezes sinks and so on until the the entire body of ammonia is frozen solid. In principle, if you had ammonia sea in temperature ranges analogous to water on the earth, the entire ocean would likely eventually freeze solid and with it the planet.
So, to start, if you want oceans on your ammonia world, would have to be relatively warm and uniformly so as ice formation would be very dangerous to the entire ecosystem. A possible way around this problem would be to postulate that the planet has very hot core like Europa and therefore ammonia ice that sinks, melts as it descends. That would also provide a lot of energy to the ecosystem even if the planet is far from the sun.
As noted by twelth, Ammonia forms a lot of stable complexes with many metals so likely any ammonia oceans would very complex mixtures or pure ammonia and various ammonia compounds. More interesting, some of these compounds are immersible to each other i.e. they don't mix and instead form layers when tossed together so an Ammonia ocean might have various layers, bubble or pockets of vastly different properties.
Now just snowballing but highly electrically conductive water masses could provide the basis for life forms that move electrons directly, as current instead of using long chains of chemical reactions hand offs e.g. the Krebs cycle.
Thermal plumes in the deep ocean could drive separation of charges by moving vast masses of conductive ammonia metallic compounds which could create the electricity to that form the basis of the ecosystem much like sunlight does on earth. Also, energy imparted to compounds that the heat breaks apart and reforms would also eventually be released electrically.
An organsim that moved electrons directly could could absorb and expend a lot of energy even at cryogenic temperatures. Instead of something sluggish as a glacier which you would get with cryogenic chemical energy transfer, you'd get something cold but fast, likely something working like a superconductor which gets more efficient and faster and deadly as it as it gets colder.
Whole different class of critter from your standard bags of carbon filled with water which move at, least face it, the speed of diffusion
Such organism would likely have fewer cells or compartments as they would not need as many chemical isolations pockets. They might be collections of giant i.e. nearly visible cells. Since moving electrons is the their primary form of mode, likely all the cells are long and fiberous. The creatures might appear to be made of woven strands of neurons with ammonia-metalic polymer membranes. Physically appearing relatively simple, they might give the vive of simplistic rag-dolls compared to complex earth life, their complexity would lay in their invisible electrical fields and circuits formed on, between and and inside their giant cell membranes.
If all the water masses are conductive possible with various immersible channels routing currents, then likely the land biosphere might evolve as electrically connected as well. On earth, its been argued that life on land more or less dragged the sea along inside it. The same basic phenomena would wire up the land biosphere into the planetary circuit as well.
The entire biosphere might resemble something more like a planet of self-reproducing robots always on the lookout for current to tap and steal. Instead of eating prey for the energy in the chemical bonds of the preys flesh, they would just short the prey organism and drain its charge taking little or no matter from the kill. But shorting the membranes might cause the giant cells or tissues to just fall apart leaving a dust of raw materials.
Good story potential. Usually the idea of organic life forms posing any serious threat to a high tech space ship and crew landing on a planet is silly. We snuffed the earth's megafunga with pointed stick and the most bad ass predator that every walked the earth wouldn't last 60 seconds against your typical Marine and couldn't get past the least metal barrier.
But a critter in an electro-ammonia based world all in spooky perpetual twillite far from any sun.
A ultra cold environment that makes metals and plastics brittle,
Organisms that have no circulation, and possibly no real critical
vital areas that sharp sticks or bullets can poke holes in.
That moves at electrical and not biological speeds,
that has possibly actually armored metallic flesh
Whose strength is determined by voltage and amperage instead of muscle so the more juice it gets, the stronger it gets.
Which can both absorb and project electricity
which will likely have have radio or magnetic based senses senses
That might be adapted to short out electronics and jam radar and radios.
That sees a human in a space suit as a walking battery for lunch
and sees the spaceship as an all you can eat buffet.
Well, now that that would make that all acid-for-blood critter Ellen Ripley had such a tussle with look like bit of a pansy wouldn't it? That little fluff ball just chased humans around the ship, it didn't try to wreak the ships systems, drain its power and maybe absorb its hull destroying all hope for survival.
The electro life form would likely completely ignore the humans but would head straight for the technology that makes us humans badasses instead of frozen meat-bags on a cryogenic world. Metal, electricity, plasma weapons (Plasma though hot conducts electricity) etc wouldn't be impediments to the creature but food. The more high tech you brought to the planet and whipped out for the defense, the stronger and more attracted the monsters would get.
They might not even notice the humans but if they humans couldn't stop the creatures from ripping apart their space suits, draining the ships power or tearing it apart for pure metals, the crew would die just as horribly as if the things actually tried to eat them.