OK...so you're saying get a 40w poly panel instead of a 20w amorphous panel of the same surface area. No problem with that other than price and complexity of installation...but a longer life and more power too.
Compared to most boat projects a 40 watt , or even an 80-100 watt solar panel (if you have the space) and charge controller are relatively cheap, assuming you are somewhat handy and plan to do it yourself, and buy mail order from solar panel supply companies not marine supply companies, I bought my 55 watt panel as a cosmetic blemish for only about $200 from sunelec.com a few years ago, and will likely buy from them again. A 30-40 watt panel, charge controller, wiring and mounting materials would probably cost under $250 and a 100 watt panel could probably be installed for only $550 or so. One thing to remember is solar panels keep getting higher and higher engergy densities, one of todays 50 watt panels is the size a 20-30 watt panel was just a few years ago.