so guns are pretty much guns here.. everyone will have a recommendation on what and why... I would take 2 very specific qualities into the process:
1) synthetic stock, you will be wet and I have had swelling in wood stocks in the environment you will mostly be in, butt in mud or a puddle leaning for hours is the devil! (but your call)
2) salt water/ rust resistant components... modern stuff has a dip coating on exteriors that mostly is camo, this helps greatly with the durability in a saltwater environment. Bluing takes a beating in salt~!. Your internal components need constant attention, if you go semi, don't forget the spring inside the buttstock. easy access for disassembly and cleaning. My SBE2 takes a shower with me every return trip from a duck day, then over a register, dry and oil.. never an ounce of troubles...