Are there any successful legal comparisons between liveaboard tenants in a marina and tenants in a trailer park?
Not sure about the legal aspect but there are several unflattering parallels that leap to mind.
On a more serious note, it's highly unlikely that there is any legal recourse. There may be a faint hope if your docking agreement specifically states that you are leasing the slip with the specific intention of living there and if the boat is your primary residence.
If that marina is indeed your only docking option it may be worth trying to fight the increase, but realistically, the legal fees are likely to far exceed the cost of any increase in slip fees.
If you are not happy, the great thing about a boat is that you really can just pick up and move.
Good Luck !