Oh boy, something I know a bit about. I've been a woodworker for quite a while and while I cant tell you exactly how to do it, I can give you some tips and also point you to some resources on the web where you can get more information.
Pauls post was pretty good in whats involved, but here are some things I would point out:
Tools: you will need access to a tablesaw or radial arm saw to make the shallow cuts (kerfs) in the piece that gets bent to follow the front of the tank. You could do it by hand, but that is really going to be tedious. I would use a bandsaw to make the curved cuts for the top piece.
Materials: I would use a veneer plywood for the bent front since it will look the best and use a solid piece for the edging on the top. Don't use liquid nails for anything like this, rather a good waterproof exterior glue. A polyurethane glue like gorilla glue would be my choice over a yellow glue.
Use brass or stainless screws in assembly and seal em up well with a plug or putty . The saltwater will cause any other screw to corrode and stain the wood, even under a finish.
A good exterior varnish will work, but make sure you do all sides and give it time to dry and out gas so the fumes dont affect the fish.
You can check these woodworking forums for more advice or if all this seems too much, you might find somebody local who could do the job for you:
http://theoak.com/cgi-bin/forum/general.pl
http://www.wwforum.com/cgi-bin/forum_main/ptools2.cgi
http://www.forums.woodnet.ne
BTW, I have a 92 bowfront and if I ever get around to building a top for it, I'll post it, but its not high on my to do list...