A cubic foot of water weighs about 62 pounds. A container 5'x5'
with 3' of water would weigh about 4,650 pounds. The load per
sq.ft. would be 186 pounds. If you weighed 186 pounds you put
about the same load on the floor.

A more accurate approximation would be:

You an 25 of your closest friends huddled in that same 5x5 area.

Weight on a 1-sqaure foot area is carried by more than one floor joist. The
subfloor spreads the load. Blocking will hel transfer load to adjacent
joists as well but I suspect the OP will need more than just blocking.
Normal residential construction (in my area) calls for 40 lbs/sf
on the 1st floor and 30 lbs/sf on the second.

-Steve



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