I use the multi purpose automotive grease for under half the cost of the anything with "Marine" or "Boat" on the label, and change the grease twice a year.
If you change it while the bearings are still warm from a long road trip, I feel it does a better job of pushing out the old warm grease that has a lower viscosity than the new ambient temperature grease.
I never just add grease because it only takes just a few minutes longer to change, and since I only wet launch & load, I want to know if I've got a seal going bad.
I think this is what I've been getting from Gordon at CFM for $4.
http://www.autozone.com/greases-and-gear-oil/lubricant-grease/valvoline-14-1-oz-399-728-g-multipurpose-grease-tube/232567/?_requestid=11239335