Tim, a line lock would be a reasonable idea for a boat ramp, though I wouldn't want to leave it for long.
These units are basically a selectable check valve installed between the master & brake cylinder(s). When you turn it on, the check valve is allowed to function and when you depress the brakes it holds the pressure on the brake cylinder side, thus holding the brakes applied. Then when you turn it off, the check function is disabled and a functions just like a section of straight brake tube - no restriction to flow in either direction.
These are very effective for holding brakes when you need all 4 wheels held on with max pressure - ie winching, utility trucks, tow trucks, etc. The problem is that eventually the pressure will leak off, allowing the vehicle to start rolling.... not good. It's also dependent on the service brake system and booster for function - so it's not a true emergency brake.
Also note that on a dual master system like most of the idis, you actually wind up with 2 separate line locks - one for each circuit.