If the panel on your boat is flat where you want to locate the metering, install the lid of a small Tupperware container on that panel. Install meter onto/through lid. Drill small hole for wires on container and lastly, snap on the rest of the Tupperware container. This will offer a near watertight enclosure for meter.
BTW, Victron battery monitors are round, work well for all functions DCV, but do require a shunt and are not totally waterproof. The price is around twice the cost of a decent hybrid marine battery. That may be a good ROI if sloppy battery charging or maintenance is a possibility. The monitor function would be a good deterrent for having to paddle off the lake.