I designed an entire floor of one of my friends house with an underwater-theme - things I used here are:
- fishtanks (obviously, can have up to 10 of them now)
- otter-lamps
- wall planters
- potted spider plant
- various Crystla Boules
- Exotic Throw Rug&Dead Mans Chest (you know, sunken treasure an all that stuff)
- various bathtubes
- I've been using some Far Eastern Doll Displays, died grey and turned around so that you cant see the markings anymore to create some kind of "coral riff"-feel, with multiple levels - however you probably need to pay some real money for them by now, since they were a seasonal item
Some other items I could see work:
- bamboo flowerpot (specially in combination with a bathtub, in which you can hide the flowerpot part, making the bamboo look a bit like some sort of seagrass) - you can basically do that with all potted plants
- Marimo Lamp
- Il Mheg Flower Lamp
- Oasis Wall Mounted Fountain
- Shower Stand
- any stuffed animal that fits the theme (Namazu, Kojin, Fuath)
- there are several pictures unlocked through the sightseeing log that show under-water-scene
- the pudding table and rug can be died blue and look kinda "watery" then - just gotta hide the pudding head and the pattern on the table top
- I cant find the name of the item but this new plant that can be bought with FATE-gemstones in Rak'Tika might work aswell
- the Eulmore marble flooring looks like it could work well as floor btw
This website:
http://en.ff14housing.com/ aswell as the ingame housing catalouge, allowing you to preview any housing item ingame might help to find some more stuff that fits your theme!
Oh and even if you cant get your hands on the doll shelves I mentioned, I'd feel that getting this more vertical feeling into an underwater-themed room is important (unless you want to go down to basically the seabed, its not really just flat down there) - so using the loft and/or wall-mounted shelves with little fishtanks, crystals, plants etc. on them might help with that!