If you can charge it at .1 amp it should charge in about 2.5 hours. If the battery gets warm it is near full charge. (or bad)
I have used a 300 mah 2s lipo in mine. It came with no battery so I can not compare to the 7.2 volt but the 2s lipo works fine in mine. ( it is a Losi battery from a Micro rc car)
i have only run mine a little so I am not familiar with all the characteristics of the boat yet. Don