I have the exact same request. I'll be watching this thread....
My thoughts were - that big steering wheel on a boat (not sure what it's called, haha), nautical stars, anchors...she is adamant that she doesn't want "sea" themed (ie: seashells, dolphins, etc.).
have you gone to the gallery and seached "nautical" i just did and there are some pretty cakes in there!!! I love using that for inspiration when i'm stuck! Good luck!
eta: i like this one from CakeL8T, on a smaller scale for the bridal shower i'm sure!
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=180853
I actually had this exact same request. I found a pan shaped like an ocean liner at my cake supply shop. Made it on top of a sheet cake and it went over great!! The girl throwing the shower got me a logo of the cruise line and we worked that into it. Unfortunately, I didn't get a pic of it! 
Since getting married is also "tying the knot", I'd be inclined to do a cake that inclued some white "rope" knots and some nautical flags--you can actually spell out words with them.
http://boatsafe.com/nauticalknowhow/flags.htm
Just a thought.
Rae
A few months ago I did a baby shower with a nautical theme. I found a really large anchor mold at the local cake shop. Also found a bunch of seashell molds. Made the anchor and shells out of white chocolate....placed anchor on lollipop stick and stuck it in the top tier...places shells all over the rest of the cake.
I made a sailing cake for my son's pre-k. I made little white and red life preservers, and used my clay gun to make a rope to go through them. I painted the rope gold with luster dust.
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