You would definitely get more range from a 25 watt fixed mount VHF than from a hand held.
But, that said, antenna quality and height are very much the controlling factors for range in a line of sight situation such as VHF. With your antanna at mast head height, I suspect that even with a hand held unit you will have more range than the typical fixed mount VHF setup with the antenna only a few feet above the water!
Silver Duck