The VHF Marine Radios (generally) operate in a Freq. range of 156. and 157. MHz. VHF FM. These radios are generally not designed to put out more power than the way they come from the factory doing. They just are not made that way. However adding more power to these radios is very easy, all you need to do is get a VHF linear amplifier such as what ham radio operators use for 2 meter VHF FM, you can find these amps in all sizes and shapes with many different input and outputs capabilities. But be sure that the linear amplifier you get does include the 156 and 157.MHz frequencies. A lot of them are very broadbanded and will work between 140 and 170 MHz. But be sure to ask around and make sure or it will not work.
Concerning the antenna, unless you really boost the power up (way up) you will (should) be able to use the same antenna..... Happy sailing.......