At least they lived to tell thier tale, sounds like this could have ended up allot worse...
I won't comment on why they decided to do what they did, but damn !!!
I am in a smaller rig now and some days you just need to stay home, drink some beers and throw a steak on the pit ...
Fastmerc, to answer your original question, I would run a similar rig back in High School (15' Monark with 25 Mariner) from Bayou Black (Terrebonne Parish) to Willow/Myrtle Bayou and occasionally out to Halters Island on a 6 gallon tank with a smaller 1.5 gallon "reserve" tank on board... It was a 50-60 min ride to our camp @ around 3/4 throttle, which was probably around 20-25 mph... 210 to the jetties and back is probably do-able on a nice day with 6 gallons, if you keep your foot out of the throttle....