Typical causes that develop leaks are lack of inspection and cleaning of the caliper slide pins, when a caliper hangs up on its slide pins it can lock at an angle against the rotor allowing the rear boot to extend to far and blowing past the rear boot as well as causing the pads to wear improperly. Other causes are due to corrosion depending on the brake system. Drums systems are susceptible to salt or rust buildup on the wheel cylinder piston which works itself past the dust boot and destroys the inner seal. Disk systems are susceptible to salt and rust buildup on the caliper pistons and inner cup as most marine disk brakes utilize a steel cup and brass insert or steel cup and stainless insert.
A yearly bearing pack / inspection allows to catch these issues before they develop into an expensive repair.