I have a nice lawn tractor very similar to yours, 18HP Kohler, cast iron front axle, hydro tranny.... I wouldn't think of towing with it.
I have a stable of garden tractors that I wouldn't think twice about towing with, I do it all the time. If you plan on doing this often I would pick up a good running garden tractor on ebay, local dealer, or scrap yard. (Yes, people scrap good running garden tractors.)
If your lot is not level look for one with good brakes. My Case has dual rear disk brakes. Your Cub probably has a small disk or a band brake. That's fine for mowing and light towing but your boat will overpower it.
I normally use the low range on whatever tractor I'm towing with to maintain the highest level of control. I don't need speed, control and safety is more important to me.
Most good garden tractors can handle the tongue weight so long as it doesn't go beyond 500 pounds. People put weights on the garden tractors for better traction. You might even need front weights if the tongue weight is high.
If your tongue weight is more than 500 pounds I'd build the wagon style hitch mentioned earlier. If you go wagon-style go even slower since you have added another axle into the mix.
Oh, how much will a good tractor cost? I purchased a nice 1970s Wheel Horse for $250 from a local dealer. My least expensive running garden tractor was $125 for my Case,(scrap yard) it needed some welding but it's a nice strong tractor. Depending on where you live ebay tractor run from $100 to $1500, decent runners should be around $250-$500.
my 2 cents,
Stan in MA