That calculator is pretty neat, but as a general rule I would double the range for an antenna 6' off the ground, assuming the other antenna is 6' off the ground.
I used these alot as a kid living on the coast. Most of us had handhelds, they were quite popular for a number of reasons over installed units.
Thats about the range we used to get between handhelds on a clear day, about 6 miles, and the actual power radiated from a 'rubber duck' with 5 watts going into it not much more than 1watt.
When talking to someone with an installed unit and fiberglass antenna from a handheld typical range was 10 to 12 miles on a clear day, regardless of if they were using 25 or 5 watts power.
Bad weather would cut your range in half, but almost always you could reach the coast guard in an emergency.
I have put up antennas for the coast guard, they like them high up as possible, sometimes tens of miles from the water, typically 400' or higher up on a tower. The setup was nothing more than a commercial motorola unit setup to the coast guard frequencies. The engineer told me it only put out 25 watts of power.
Moral of the story is, on VHF you are limited by your line of site. If with a theoretical telescope you cant see the top of his antenna from the top of your antenna there will be no communications.
If the top of your antenna is at 10' and the USCG is at 400', if the formulas of this horizon calculator are correct, then your line of site is 4 miles and his is 24.5, then the line of site from antenna to antenna is 28.5mi, however this will be obstructed for the most part by the earth itself, so after 26 miles (line of site distance from the bottom of the antenna), the signal will rapidly degrade.
High power is great for blasting through rain, humidity, pine trees, buildings, talking over interference and getting the most out of that last 2 miles of range before your line of site is completely gone, but its not going to get you more direct range than line of site.
Sure VHF can go hundreds and even thousands of miles, but thats if it skips because of atmospheric phenomena, and thats if, and then, just as the name implies, it skips, as in skips the local coast guard 40 miles away and ends up coming down and being picked up by the local coast guard 400 miles away who aint got clue where you are at, and thats if, skip is not a reliable way to communicate, its more like a rare phenomena with vhf FM.
Up on my mountaintop, with a 30 watt 2 meter setup and a 5db antenna, I could hit repeaters 60 miles away even in the worst of weather. I was 3000' agl and their repeater was 2000' AGL, and I could always hit it loud and clear.
I think buying an amp would be a big waste of money. If you cant get the ranges I described (over water not land), something is wrong with your system.
Scrap the RG8, get LMR400, its not expensive for such a short run as on a boat, and its much more robust and durable than RG8. Have a radio shop do it if you are not 100% confident you can route, ground, connectorize, test the coax and tune the antenna, because from what you say this really is life or death.
And heat shrink or wrap those connections. Cant stress this enough. My work is often up on a tower, and I see connectors fill with water even when wrapped or heat shrink, because if done poorly even it fails, o-rings dry rot and fail on "water tight connectors", barrels unscrew and water seeps through! Just a little bit of water will cause most of your power to 'reflect' right back to the radio instead of the antenna, so will a kinked line, a poorly done connector or mismatched (poorly tuned) antenna, but water tends to be the worst. It often results in a radio failure as well.
Once again, spend the money on a good install instead of the amp, 25watts is plenty for over water.
