I used large concrete blocks, half-filled with more concrete to hold a stainless U-bolt, and stainless chain up to eye bolts on the platform. Then I used those things that mountain climbers use, I don't know the name, but this lets me adjust the chain length at the pond goes down so the dock doesn't wander too much. Anchors are heavy as heck out of water, probably overkill. The strongest windstorm has yet to move the platform.