"Peter Hendra" skrev i en meddelelse
...
Hi,
A fellow boater here in Trinidad has rusty steel water tanks with a
snip
If so, what would be the most cost effective (cheapest) internal
coating?

Coal Tar Epoxy not a good idea.
A temporary fix might be a cement wash. Never tried it myself, but I believe
it was a commonly used treatment in "the old days".
HTH
Bob Larder