I think your water would have been significantly warmer.
Pond water with a thin veneer of ice on top would still be quite a bit warmer than -3˚C ocean water. People very regularly swim in lake water that has thick ice on top of it (after they cut a hole through).
> Pond water with a thin veneer of ice on top would still be quite a bit warmer than -3˚C ocean water.
Specifically, it would be pretty much exactly 0˚C at the top and 3.98 °C at the bottom. How thick the ice is does not matter. Whether than counts as "significantly" or just "slightly" warmer is a matter of opinion.
I think your water would have been significantly warmer.
Pond water with a thin veneer of ice on top would still be quite a bit warmer than -3˚C ocean water. People very regularly swim in lake water that has thick ice on top of it (after they cut a hole through).