lets start with the basics
**What would an aquatic race want**: remember it needs to survive underwater or they won't want it. They won't have much use for clothing water is not an insulator like air so clothing does not help much in maintaining warmth and there is no rain to keep off.  

**Glass** especially mirrors, bottles, and lenses no way to make glass under water. glass floats will also be popular. 

**Brass, copper, and steel**: steel is actually alright underwater as long as it stays underwater, it will rust faster than on land but will stay usable for decades. . tools and weapons will be especially popular, even brass nails will be valuable. 

**Lumber** especially poles: very little wood underwater and again as long as it stays underwater it will not rot any faster than on land. 

**Gold**: again no way to smelt metal underwater, they can work gold but have no way to smelt it. silver will become quite ugly very quickly however. 

**Pottery** plates are useless but jars are easier to make on by shaping and firing than carving as aquatic races would have to do.

**Surface foods** food has always been a popular part of trade, this will run both ways and will be most common where both civilizations are close to each other. Cooked food will be popular, every animal ever tested shows a preference for cooked food sue to the higher nutrient availability. 


Now for the aquatic races **What can they offer**.

**Sponges** sponges have always been valuable enough for trade and they are far easier for them to collect than fishermen. 

**Ore** there are ore deposits underwater just as on land. Many are actually easier to collect.

**Fish**both as direct trade and as indirect by selling information about movements

**Pearls** this one should be self explanatory

**Maps** seafloor navigation maps take a lot of time to make, an aquatic race can make them much faster and cheaper, they will need a human agent to copy them to air friendly materials. 

**sunken ships** the location and recovery of sunken ships and their cargo will be profitable and fairly easy. This of course could create pirates.

**Art** coral carvings, shells, and similar art will always have a market. 

**Ship maintenance** certain aspects of ship, bridge, and dock maintenance will be much easier for an aquatic race this could easily be worth trading for. 

**Travel rights and guards** through territories/kingdoms will be a big deal, for those aquatic races that are amphibious this will run both ways. Remember they can sink ship with ease if they want to. I could easily see a negotiation for guards for ships. who better to stop merfolk pirate than other merfolk.
 
There will be other things they can offer all depending on how they live and what their technology is like. maybe they make a seaweed extract that is really savory or collect a snail venom that is useful for treating cholera. 

I can picture trade houses built out of stone that have a waist/chest high wall with water on one side and dry land on the other. trade partnership between merchants should be common. Math and standardized measurements will be important, a pound of iron on land is not a pound underwater. 

writing will probably be exchanged each way just for the novelty value. 

I imagine their written language will be runic relying on carving rather than ink, I could also see a knot based language like Quipu.