I agree with what Dennis posed above.
I’d like to add a bit of reality. An installed marine unit will cost you about $3,000 if you install it or about $4,000 if you have it done. If you do this, keep in mind this will add very little to your resale value.
The over the hatch unit portable AC unit Dennis mentioned is an option. About $1,000 new or ???? used.
Another option is something called a portable air conditioner, about $300 brand new from Best Buy or similar stores. Advantage is low cost. Disadvantage is it takes up space inside your small cabin. You vent the clothes dryer style vent out a port window when you use it, take the vent down when you are done.
As Dennis mentioned above, all of these options only work at the dock unless you are going to add a marine generator, cost for the generator with installation runs about $9,000 - $11,000.