Regenerator numbers don't mean anything. You're still getting the same amount of Regenerator recovery that you would have got before, only now you have one less HP point for...reasons. Same goes for dropping HP for Leftovers numbers (a concept that I wish would go die in a hole). The only reason you should drop HP most of the time is to reduce passive damage. Toxapex's max HP is divisible by 16, so 248 HP would work for that purpose, but it also reduces recovery from Leftovers and Recover as has been pointed out already. At that point, you have to ask if the extra point of recovery is worth the extra point in passive damage. Seeing as how Toxapex is immune to poison, Stealth Rock only affects it once per switch-in, and Tyranitar/Hippowdon aren't exactly tearing up the usage charts right now, I think the extra point of recovery is worth it in the current metagame. Plus, Toxapex's base defenses are so much higher than its HP that the extra point of HP is more efficient.