Which charger do you have? What kind of prop is on the boat?
I sometimes hate to say this - but spend some more money and get some new batteries with a higher MAh rating and a new charger.
I have a Dynamite Prophet Plus LCD
charger and routinely charge 3000 MAh packs in 35-40 (4 Amp charge rate) minutes at the lake. I also get around 30 minutes of running time in my Dumas Chris Craft Dual Cockpit runabout with those packs. I'm running an older Dumas (pre Raboesch) 3 blade 1.5" cast bronze prop, direct drive to a stock 540 size motor.