Pretty weird case of a planet, especially if you haven't seen one first hand.
IMHO it could go like this (but would probably be different in most cases out there):
- 1 water planet with a highly elliptic orbit around a red dwarf - that could ensure there is no tidal locking, but no guarantee it'd stay so so for billions of years; the orbital period would be quite small, as the red dwarf is a small star so the planet would be quite close to it
- the planet looses atmosphere over time, and also water; after a long time it would end up with an Earth-like ocean, with land above water
- as the planet moves closer to the star tidal forces heat up its interior; volcanic eruptions may ensue; these ensure there are other gases in the atmosphere besides water vapor
- the atmosphere could be quite thick; this, combined with oceans could give a climate inertia to it, i.e. the planet would retain heat even in the far reaches of its orbit, over days or weeks as it moves away from the star.
The end result would be a planet with plenty of liquid water, a thick atmosphere composed mainly of water vapor but also some carbon dioxide, hydrogen sulfide and nitrogen, with plenty of rain, with volcanoes even long after it reaches Earth's age (that is if it manages not to get a circular orbit and a tidal locking)... It could be quite a violent place, with weather extremes. There would be a summer and winter cycle, which could have mild or strong climate differences depending on atmospheric insulation, distance from star and how elliptical de orbit is.
There could very well be tectonic plates due to the presence of water (it improves plate slipping at the contact between plates). And as I said if the planet avoids somehow to get tidally locked the tectonic plates could run for longer than they would on Earth, maybe even for trillions of years.
If there is an ecosystem, it'd probably evolve from water-based to land-based. It could last for trillions of years if it is lucky. But that depends a lot on how many supernovas go on near it on that time scale.
So the end product would be a somewhat earthly planet, with a very long dynamic history and which would change significantly over large time scales due to atmosphere runoff. It'd have a sort of superseasons over billions of years, going from ocean to Earth-like to dry as all water is lost.
But then it could be something completely different. The best thing is to see one for real to figure how it could look.