I recently just put together the most stunning crib set with a nautical theme. It's very classic in red, white and blue minky and ticking stripe. We accented it with a simple navy, nautical ribbon....very classy! Once it's made I will have to share it with you so you can see how fabulous it is! So after putting the set together I was inspired to create a child's banner to go with it. I started surfing around to find some inspiration. I enjoy the Better Homes & Garden http://www.bhg.com website where you can get all kinds of ideas. There I came across this room by designer Jodi Mensing.
The wall treatment is something anyone could do but I think the beds steal the show. If it's not in your budget for custom made beds you can still get a nautical feel by using the following details.
You might try fishnet, rope, treasure maps, life preservers, and flags! I've also found unfinished wood paddles and anchors that you could paint yourself at Michaels http://www.Michaels.com (another one of my most favorite stores). I also suggest using red or blue ticking fabric for window treatments. You could thread rope through large grommets to hang.
So if your looking for a fun little boy theme, you may just want to go Nautical!