I know, I know, there are tons of posts like this out on the internet. This one, though, is a compilation of those with new snippets added from an actual artist who gets her ideas from her whacked-out brain, not others. So there's new stuff. Hang in there if you don't see it at first.
First off, let me say how @#$%-ing much I love underwater themes. I'm far from my native home, which was by the ocean, in a land-locked state. I try not to think about it too much, and my bedroom is my favorite place to relax and forget. I'm always trying to add new elements to make it more "sea" like.
White crystal lamp (or nightlight), fitted with a blue bulb inside. This is great because it's got a great underwater look to it, plus the soft blue glow adds to the blue water ambiance of the room. The white crystals are more fragile, BTW, so be careful.
Murals are awesome ways to add the sea to your room. You can get ones like wallpaper murals where you have to glue them to your wall (right) to cheaper, picture ones that come in many different pieces of paper you can tack up (left). The one on the right is a Komar Wall Mural entitled "Atlantis" that had to be pieced up and glued to the wall by myself and my dad. The one on the left is a photo mural, composed of 16 or so pieces of photo that we placed on poster board and then tacked to the wall to make it more sturdy. Incidentally, there's a stuffed jellyfish hanging from the ceiling shelf that I have wrapped around the room to further create an illusion of being underwater. Stuffed jellies are great, there's tons available for sale now.
Accent Bottles and Shells are a must have, but though I'd think they're the easiest things to make, some crafters out there sell them for up to $50 a pop! I don't know how they find buyers, since let me honestly say I can't (and money being what it is, I've tried). But let me ruin things for them here by saying: you can easily make all of these things above in the photos. Very easily. I mean, it's ridiculous how easy it is.
* Take an old candle jar, for instance, from Yankee Candle or something, peel the labels off, wash in the dishwasher and scrape the gunk off from said label(s). Fill with beach sand from a craft store and a few shells. Done.
* Take a jar with a lid that fits snugly (beer bottle, glass milk, etc) and fill with a solution of glitter-glue (blue colored, of course) and warm/hot water. Add more of the glue than water, stir, drop some shells, stones, and/or a rubber fish toy like I did here inside. Fill as far to the top as possible and superglue the lid back on - viola! Glitter globe bottle.
* Find blue glass bottles and such at garage sales, flea markets (even beer bottles like Bud Light Platinum!) Fill with shells, etc.
* Marimo (moss) balls are even better than betta fish for live accents of the sea in your room. You can buy them off eBay for much cheaper than a pet store and plop them in room temperature water in any bottle or container you have laying about. Roll them about once in awhile, make sure they get indirect sunlight and you're good to go!
Outlet covers/light switch plates are easy to find with sea motifs on them. Just make sure you get real baked-on ones and not ones that folks just glued pictures to. The quality is so much better!
Blankets/quilts for your bed may seem a simple matter, but I've found that finding sea-related sheets and whatnot is really hard. My mother quilts, so I always have wonderful covers for my bed - I'm lucky. But if you don't have access to a crafty mom (and can't do it yourself), a low-cost solution is a throw blanket. Mine's queen-size, so I paid around $50 for it, but there are others for less out there. It seems that sea turtles/dolphins are popular for these, so you can find a few patterns and pick the one you like best (I found one with a reef on the bottom after wading through quite a few wolves, horses and NFL teams, lol).
Here's the best tip of all, though: aqua-colored curtains.
I bought some sunlight-blocking aqua colored curtains for my room, and it was the best purchase I'd ever made. The room is bathed in an aqua light, like being underwater, and all from such a simple thing. People make comments on it all the time on how cool and relaxing and "underwater" it looks - all from the light half-blocked from the curtains and coloring up the room.
There's more, of course. You can buy many things to accent your room, such as "wave pots" that create an illusion of waves on your walls or ceiling. There's fish plaques that look realistic, and toys that are, too. You can always add netting and hold your stuffed animals up in it (if you buy fishing net it adds a bit to the decor, instead of a "petnet").
Whenever I get more ideas and add 'em, I'll add them to this post. Hope it helps you out there!