I've got a pair of Marine Machine sea stainers that the lids are seized. The boat has been in salt water most its life, so I am sure that is the issue. I've tried using a breaker bar, two different impacts and I've had zero luck getting them off. Anyone have any suggestions?

I'd say remove them from the boat and flip them upside down and try some PB blaster to penetrate the threads or maybe even vinegar to loosen up corrosion.

I take it,the hex head is a bolt that goes all the way thru?if so,your probably gonna have to take it out of the boat.then either try heat in the area where it threads into,or,cut the head off.soak with acetone and used tranny fluid mixed together.maybe you can get heat in there too.
salt corrosion is some strong azz schit.

New strainers! Sounds like they weren’t opened for quite a while. Be careful of what might be living in there when/if you do get them opened. I’m systematically going through and replacing salty worn parts out of my boat as well.

I can tell you how I removed but you might not like it. CAREFULLY take hacksaw and cut side of lid. Be very careful when blade gets close to threads on strainer. Once slit in lid is there, take blade screwdriver and twist slightly to expand lid and remove lid. Repair lid slit with epoxy, sand, paint and reinstall with anti seize.

Are these the ones you have?Try rust bust stilsons wrench may work?

I know it is very easy for us to spend your money, but if I were in your shoes I would look for new Strainers as well. If you are successful in getting the lid off that would of course be the best outcome. but remember it must seal up tight when finished as well.

The problem is that it is an aluminum lid and a stainless housing. The ole dissimilar metals thing at work... I had the same problem on my TopGun. I can't remember how i got them off, but I did. I then had to buy new lids because I phucked them up getting them off.... LOL

Try heat also not to much!

I used heat in my old mistress and got them broke free. I put anti seize on the threads every year now in my top gun. Good luck!