Actually in my experience, nothing works as well as a big leg. I shoot in water about 70% of the time and fully understand the issues you are having. I use a RRS 3 leg, 3 sections per leg tripod in water, and I now use the rubber feet 100% of the time. They don't grip as well as spikes, but do give you some shock protection. The RRS with center column have a hook (which most tripods do now) that you can attach a heavy bag (just like Nancy P suggested), I also carry the grapefruit bag and fill it up with rocks. Hang that from the center column or if you don't have the center column, under the base should have a hook. This will give you a lot more stability.
I just switched to a Gitzo Series 2 4 sections per leg, tripod. Not as heavy duty as the RRS, but much lighter and I can carry it on longer trips. The RRS with BH-55 was just getting a bit heavy for me. The Gitzo is the 1542 model.
If I am closer than 1 mile, I still use the RRS as it's a great platform. But if there are a lot of hills involved, I now go with the light tripod.
Paul