I have a lifting one with a spring, however it also locks on a screw (the spring is to aid lifting only). Its like the ebay one but without the mounting plate angle adjuster (which looks good actually!). It may be fine for a yacht but in rougher seas in the up position it bounces around and I'm concerned it would eventually fail. The clearance on the bolts that form the pivots for the linkage is enough that in the fully up position there is free play. My fix has been to lock it in the half way up position so that the linkage makes a rectangle, not a diamond, and is much more rigid. I then chock the aux up with a bit of wood and strap it down solid.
I have yet to see what I think is a suitable lifting aux bracket for a powerboat. I don't fancy lifting an aux onto the transom in a pooh/fan moment, so go with the best I can find.