The $4-5k is the hard part. I'd say a 2011 GTI 130 which can be found for a little over $5k. The IBR and throttle by wire are incredible technology for a low price not to mention having a brake that works very well. The other I would get an older RXT, like 05 or so which can be found around $5k as well. It is SC, but the power, acceleration, and top speed are worth it. Just plan on the rebuild and is not a big deal once you accept it and budget for it as normal maintenance.
What not to get? A Spark or an older two seater. If you ride in mostly rough water like you stated, it will beat you up pretty good. Unless your 18 years old and like constant jumping and being tossed around, you may not be happy with your purchase. I guess I'm just getting old.