1500 feet is all that is the norm for RC radio gear . For anything else , you will need to contact the FCC and license yourself for ham ( AND then ,hope they give you a temp permit , which I honestly doubt they will do for the reasons you are wanting . )
No matter how you throw the dice , I think your biggest problem is gonna be the radio gear needed for your intentions .
Can a radio be modded to amp out the signal ? yes . Can it be done for the distance you are wanting ? Yes ...
Can you find someone that will DO that mod for you ? NO ... that is TOTALLY illegal .
750 milliwatts doesn't get you too far and the closer to that 1500 foot mark you get , the greater the chance of a glitch and then ... out to the deep blue for your hard earned monies invested in that boat and setup .
I agree with the others , it wold be cheaper to just get yourself a regular sized boat , grab a 6 pack of pop and start dropping your line(s) ....
Oh ... and take a snooze too , if you'd like