I've considered buying a 509 a couple of times, the magazine prices that I saw were the main issue that I never followed through.   I did see a review that brought up a potentially unique issue, when the gun gets wet it won't fire.  I understand that water has to get into the striker channel and that it's either tolerances inside the channel or the weight of the spring that cause the failures but it's an interesting issue.  If anyone has a 509 and is willing to dunk it in a bucket to see if their pistol has the same issue that would be helpful.  I don't have a sample to test unfortunately.  Please check out the video below, id be curious to see everyones thoughts.

https://www.youtube.com/watch?v=9wkQ2NOE7Rs       That link should take you to Jhelmsings video where he runs the same water test on a 509, glock 19, and p320

Unless you carry while snorkeling I think you'll be ok with FN.

Maybe, but if you bothered to watch the test you would see it takes very little water to induce a failure.  Kind of weird a company like FN wouldn't notice this during testing.  I'm not aware of any other major brand pistol with this issue

I have a 509 Mid and haven't had any issues at all.

Hydro-locking can happen with many striker-fired designs. But no matter, I’m not worried about it.  The 509 is an excellent firearm IMHO.

In the video, if you watch it, the gun doesn't work consistently even when the water has been drained from the gun.  I don't own one, I posted this to give people who do a heads up.  If no one cares that's fine, if I owned one I would call FN and ask them if they were aware of the problem.  Minimizing this issue doesn't make sense, you can run the exact same test with your own pistol if you don't believe the video.  Also, suggesting other guns can have the same issue is probably the least helpful comment that could have been made.  The tester is using a Glock and a Sig to demonstrate the problem is specific to the FN.  
This same attitude was exhibited when I was an early adopter of the P365 from sig and had numerous issues.  Pro sig people couldn't handle that the gun (an inanimate object) may have a design flaw or two that should be addressed.  Instead they said all of the issues were user error.  A couple of years out and I am simply one of thousands of people that had issues with their 365, it is only because of the customer reports to sig that they made changes to the Striker specifically.  I have a 2nd gen 365 that has over 3400 round through it and is extremely reliable. We all benefit when we let companies know that products that we paid for, with our hard earned money, have an issue that is well within their ability to resolve.

Watched the vid when it came out, tested it myself the same way, gun worked flawlessly.

Thank God I live in the woods and not the ocean

but how can you operate operationally in that environment????