Unless you have a diving board, nine-feet is a
lot of water-depth for a swim raft!
(Although distant Winter Harbor neighbors have strung
three swim rafts together—presumably to limit the proximity to their shoreline of noisy V-hull speedsters).
☻ You definitely need more scope, but there's
other things to try first.
☻ Swivels keep a raft's chain from "getting shorter" (and weaker) upon twisting.
☻ To limit the time the raft can spin, use
one chain and re-attach it to the end that faces the prevailing wind (NW in most places): or attach a second ladder.
☻ If your blocks truly weigh 250 pounds each, that would be enough if they were connected
in tandem.
☻ Your "wing-dings" may have used straight-concrete, and not added [heavier] rocks or iron to the mix. Concrete "looks" heavy, but it's not when submerged.
☻ Contact professional renters of swim rafts—
find out what they use—then
quadruple-up on it!
☻ At the end of the trailing (second) block, add a boat anchor. I would use heavy nylon line where it's buried under sand, silt, or mud. There are occasions when storms could move your raft more than a mile!
☻ You'll still need a swivel next to the raft.
☻ On a calm day, drag a garden hose to the site, and "pressure-bury" the blocks and anchor as much as possible. Attach the hose to a 10-foot length of PVC pipe if you don't have teenagers to hire!
☻ Check that the attachment of the stainless-steel chain is secured to adequate hardware at the bottom.
The proliferation of oversized boats can be witnessed with the new breakwaters formerly off-limits to some shorelines.
Even boat-lifts are appearing in Winter Harbor, which is protected from most windstorms.
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