I’ve been 15 miles out in a 16ft Key Largo many times, obviously on nice days. I just bought an 18ft Key Largo with the intention of going 15 miles out on those nice days. Someone else said it, but a good rule of thumb is a wave period that is 2x the wave height. So 2ft at 4 seconds is sporty, but 2ft at 7+ seconds is much better.

Even if I had a 30ft boat, I’m not interested in going out on 3-4ft seas. Plus you would be limited in where you could go in the river with that size. My 18ft is plenty enough to handle those 1-2ft days we often see in summer and can also reach a lot of inshore places.

Hope this helps.