The maximum power out of most VHF radios is regulated at 25 Watts. This is a FCC regulated requirement. The quality of coaxial cable used in the antenna install is critical. The longer the run, the fatter the cable should be below:
Antenna quality is also critical. Cheap antennas will radiate the 25W in a circular pattern (wasting lots of power radiating vertically, unless your talking to airplanes and submarines). An antenna with "Gain" will "focus" the radio frequency horizontally where it will do the user the most good. Buy a good quality antenna with at least 6db of gain.
Since VHF emissions are "Line of site" the taller your antenna, the further you will be able to communicate. Buy the tallest antenna that will work for your boat.
To summarize maximizing VHF performance, use the correct coaxial cable, the tallest antenna practical for your boat, and the antenna with the highest gain that fits in your budget.
Additionally 100W linear amplifiers are available. These units are not legal for use in the USA. If you use an amplifier, be sure your antenna and cable are rated accordingly.