Ok, I grew up right off of PCB so here it goes.
A. Vela it's worth it if you don't mind overpriced drinks and stay the FUCK AWAY from the pool/foam party. No amount of chlorine in the world can kill what lives in those waters. 175.00 for a week isn't terrible because cover hovers between 15-20 early (think like 6-7) and can go from there (I went a few years ago they wanted nearly 40 bucks). Also, locals stay away from Vela like it is diseased (which it may be) so there are a ton of spring breakers there.
B. Holiday Inn, Yes stay there or somewhere near there. You won't regret it even if the MTV Spring Break isn't there the location in regards to the bars is worth it alone.
C. If you rent a boat know that a. you may get fucked with by the coast guard who are out in force during this time. b. the coast guard hates fun. Worth it to go to Shell Island but honestly there won't be a shit ton of people there as that's where the locals go to get away from the Spring Break crowd best bet is to rent a skidoo out off the beach for a bit or take a pontoon and float down the beach a bit. Women will get on a boat and it's a good ass time if you do it correctly. Personally if I had to rent a boat during spring break I'd rather not. If you do decide to go down Lagoon Dr. till you see the dive shop on the left where it merges onto Thomas Dr. On the left are a few pretty good boat/jetski rental places.
D. Enjoy yourself I'll be back myself for Spring Break next week as well and I intend on not remember this shit before I head back to law school. Paddy's day is fun as hell definitely hit up Coyote Ugly's. Keep in mind that every bar will be overpriced so save your money for covers. Also if you ever just want to see a concert at Vela/Spinnakers the beach access by Vela is your friend and it's usually packed. Get the panamaniacks card it's worth it IMO and please stay off the sea oats no one needs a 500.00 fine to go home with. Finally, Pier Park is a shopping mall let no one fool you elsewise. Yes there is margaritaville and it's cool but it's a tourist trap as is most of the restaurants in there.
Eat at local joints if you can Jenny's One Stop diner (if you wake up and find yourself over by Sharky's) is great food for <10.00 that will cure any hang over. For Oysters and seafood I prefer Schooners (pricey), Sand Bar(meh not too bad), or the Bayou on the Beach (FYI the lunch specials are the tits) I can tell you pretty much off the top of my head the specials I used to eat here so often when I lived back home.