Per the National Electrical Code, Article 555.11, the main overcurrent protective device (circuit breaker) that feeds a marina must have ground fault protection not exceeding 100mA. As an alternative, individual branch or feeder circuits may be protected in a similar manner.
Of course that doesn't mean that all marines are in compliance. Of greater importance is that 100mA rating is only meant to protect equipment, not people. For comparison, the GFCI outlet in your bathroom will trip at only 5mA. That rating will protect nearly all people, and it typically takes more than 8mA of current through the heart to keep people. The greater danger in falling into electrified water may actually be drowning because you can get to a level that won't kill you electrically, but it will cause your muscles to seize up, causing you to drown.
So to answer your question, while the marina *should* have ground fault protection as required by Code, the requirements are meant to protect equipment, not life. It is not GFI protection as you know it in your bathroom and kitchen outlets. As I recall, this didn't become a Code requirement until sometime in the 90's, so older marines are at greater risk of not being in compliance.
To respond to a couple of posts above, falling in electrified freshwater is far worse than falling in electrified salt water. Freshwater has greater resistance and therefore the voltage gradient will be higher. The salt in the salt water makes it a great conductor so you have less resistance and less voltage gradient.
You do not want to test the marina's GFI. If you do that, and it works properly, you'll likely trip the breaker which feeds all the slips and have no way to reset the breaker. A home GFI outlet tester will not work anyway because it's designed to draw just 8mA of current, which will not trip a breaker with 100mA ground fault setting.
"Electrified water @ 30 AMPS" does makes sense. 30 amps may be the rated capacity of an outlet, but it does not mean that's what's coming out at any time. The current through an outlet is dependent on the voltage applied and the resistance of the load. Dropping a 100A (or even 1000A) outlet in the water is no more dangerous than dropping a 30A outlet, all else being equal.