Light fitting had water in it - bulb has been removed, and the switch is off, but breakers for the downstairs light circuit won't turn on
If there are sparks, something has gone wrong and that breaker is doing its job and preventing your house from burning down.

Most likely the power runs into the box where that light is first, then to the switch.  That's a completely normal thing.  Look up the term "switch loop" for details.

First step is to remove that fixture and check for any water damage and damaged or loose wiring - both in the fixture itself and in the box it was mounted to.  Since that's where the sparks are flying, it's almost certainly where the problem is.  Before disconnecting any wiring, take lots of photos showing exactly where each wire comes from and goes to - you'll be grateful for those when you go to wire it back up.

Then it's just a matter of fixing whatever has gone wrong.  Might just be a wire that has come loose and needs reconnecting.  Or it might be severe damage to the wiring and require replacing.  Won't know until you get in there.

If somehow that doesn't turn up any problems, do the same for the other fixture.  Then the switch.  Then anything else that is on the same breaker.