I have been asked to make a cake that looks something like the cake on here.....

http://blowoutparty.com/blog/2011/05/boy-beach-and-pool-party-inspiration/

or here....

http://frostedeventsblog.com/2010/05/04/birthday-girl-real-parties-under-the-sea/

Now--while I think these are majorly cute and fun--I have never made buttercream creatures like these. I don't know if I am confident to try on someone's cake. I know I could make something cute out of fondant, but I want to please the customer. Anyone have any advise?

Practice on a piece of wax paper. I think you'll find them fairly easy. I'm confident I could do it and I can't even write on cakes. lol

It's called figure piping.

My advice...
Search for tutorials on figure piping techniques, or view the technique on the Wilton site:
http://search.wilton.com/?q=figure+piping#i=1&q=figure+piping&type=Techniques&u1=q&u2=type

You may be interested in Chef Mike Terry's DVD on Fat Buddies. He uses a cupcake or two as the base for his critters and pipes mounds of buttercream on them to make fabulous little animals and creatures in about 60 seconds each. I have seen a couple of his demos at ICES events, and he is the master of this technique. Check out three little examples in the middle of this page

http://www.bakerypros.com/pictures.html

and then click on the "for sale" tab if you're interested in ordering. The basic techniques can be used without the cupcake, too - just pipe smaller versions onto the cake or whatever.