I have a 24' pontoon boat with a 150 HP outboard on it. My typical load is 5-6 people plus ice chest, etc. We tube a lot. I suspect 8 people on anything smaller than a 24' boat would feel snug for any prolonged period underway (e.g., tubing or traveling to a favorite spot). With that many people and a tube, you'll want at least 120 HP for the tube ride to be more than wet and boring. ... You didn't ask, but the rule of thumb for fuel consumption is one gallon per hour per ten HP at full throttle. So if you run full throttle while tubing with a 120 HP engine, you'll be burning about 12 gallons of fuel per hour. My local marina is currently charging about $4 per gallon.
