Just by the specs, with another 150 watts total (a 23 percent increase) you will pick up another 1 dB or so, or in other terms, another click or so on your volume control.
A more significant upgrade will be seen in the SYN4's improved efficiency. At equal power levels you'll have longer playtime at rest. And, when you drive your system hard, the boat's voltage is going to drop resulting in lower amplifier power. But a more efficient amplifier will draw less current which translates to a more sustained voltage supply, thus the more efficient amplifier produces more power. Plus, a more efficient amplifier is kinder to your charging system. These are benefits that are difficult to convey with specs alone.
The WetSounds SYN4 uses a Class AB output stage which is recognized for really good sound quality. What distinguishes this amplifier from other good Class ABs is the Class H power supply that is either very generous or very stingy depending on the instantaneous demands of the audio signal. This requires some pretty sophisticated and speedy microprocessing to govern the power supply that tracks and mirrors the audio signal. A pulse width modulated power supply is not generally the main source of an amplifier's inefficiency but the Class H power supply also significantly impacts the efficiency of the Class AB output section. The SYN 4 converts less power to heat (less waste). As a result a higher ratio of the power it draws is delivered to the speakers.
I would encourage everyone to place greater emphasis on more efficiency in every aspect of their audio system, especially in a boat.