If the keel breaks it's a problem.

Thanks for pointing out one type of damages that I should not attempt
to fix. I will stay clear of boats that have broken keel.

Any other damage in a fiberglass boat that is too severe for repair?

But if the keel is sound, boats can usually be safely
supported on blocks under the keel, with the hull braced
along the sides to it stays level. Take a look around
any fishing port or boat yard.

Good to know this. I think I kind of know the braces that you are
referring to.

If you are on dry land the boat can be jacked up with
bottle jacks and blocks put under the keel. You can
get a boat off it's trailer that way.

This is the part that I don't quite understand. The boat is still on a
trailer. How do we move it from the trailer to the wood blocks where we
can jack the boat up? Do you mean constructing the wood brace right
under the boat with the trailer still in place, and then jack up the
brace (in other words, don't move the trailer)?

Thanks in advance for any following-up message.

Jay Chan