I've got a '70 19-1. 6 miles is nothing in the right seas. That said, anything remotely over 2ft, especially short period and you'll be wanting more boat. I slow down to ~18knots cruising in 2's, you'll be to your spearfishing spot in 20 minutes. They are wet boats, but you clearly are not in this game to stay dry. Boat handles 4 adults and gear well enough if you pay attention to where people sit to keep the boat trimmed right.
I originally powered the boat with a Yamaha 70 two stroke, got 25kt WOT. not enough motor for me, now I'm running a 130 two stroke Yamaha and can hit 37kts WOT with a 19p prop which is scary fast in this boat. Could easily spin a 21, maybe a 23.I recommend against a 150, you cannot use all that power, and don't need the weight. A 115 is plenty!