Maybe I'm in over my head- I am planning on making my daughters birthday cake. The theme is "under the sea" and I found a great cake recipe on this site... And I've learned SO much already. Now I don't know anything about which type of frosting and fondant to use. My vision is a rectangular base cake and maybe carve the base so the borders are wavy all the way around??I have a sphere cake mold which I was planning on filling one half with batter and then putting that on the base and carving it like a rock??? Also was going to frost the whole thing to look like an ocean with waves and was going to stick a Barbie doll or something similiar into the "rock" and pipe a mermaid tail onto the rock to look like a mermaid. I hope that made sense?! Well, I have no idea if I should use buttercream or fondant and no idea on any well recommends recipes? Any tips, thoughts, ideas, criticisms would be greatly appreciated as this truly my first attempt at anything like this. I plan to expand from this experience, and I consider myself a very experienced baker aside from extravagant cakes (I know this may be anything but extravagant to some of you experts). Thank you do much

Im a fan of fondant covered cakes, something about them appears more professional IMO.
But I think if you're going to be adding lots of blue & white for water.. maybe you could slap lots of buttercream... or piping jelly for the wet look...

Sounds like you are planning for something similar to this

Maybe it will save you some work if you make the rock out of RKT and cover it in fondant. I just did a mermaid type cake and covered the cakes in light blue, then did two (different hues of blue)wavy strips around the cake.