I really like Capt_ Jordi's idea of an angler fish, I think it would be easy enough to handle. As far as getting it done;:
I like spray painting but any acrylic paint works fine. If you go to a craft store they'll be able to help you.
Don't worry about your painting skillz. As far as kelp goes a bit of irregular shaped sponge (even the corner of a dishwashing sponge) and a brush can help you create an awesome seascape. and if you don't like it, wash it of and start over!
craft foam and toothpicks are your friends.
Let the pumpkin/gourd speak to you and tell you what IT wants to be. The misshaped ones are the best!
Hope this helps. Good luck!