The actor was spotted living it up Wednesday in Tulum, Mexico where he enjoyed a dip in a cenote -- ya know, those natural water pits in caves that put your backyard pool to shame!

Safe to say the guy's getting better every day after April's health scare, which required months of treatment, and rehab and recovery. We finally started seeing him last month, out and about in the Chicago area.

Jamie gave his fans a very promising update Wednesday, saying he's "finally startin to feel like myself ... it's been an unexpected dark journey ... but I can see the light."

He said he's someone that's incredibly grateful -- and offered up his thanks to everyone that was wishing him well in his uncertain time.