The power forward -- who last played for the Charlotte Hornets -- and soon-to-be wife Alisah Chanel hopped aboard a flight and went to Soufriere, St. Lucia, for some rest and relaxation ... even taking a spa-type mud bath outside in the forest.

Once the two got out (all muddy), Washington -- who proposed to Chanel in 2022 -- jumped on the opportunity to leave his handprint on Chanel's butt ... and he took full advantage, leaving his mark on both cheeks!

The print didn't last long ... the couple washed off under a massive waterfall.

It's good to see Washington and Chanel still goin' strong after all of his relationship drama over the last few years ... which all went back to his relationship with 31-year-old IG influencer and fitness model Brittany Renner, whom he had a baby with. PJ started dating Chanel shortly after their breakup.

Many criticized the 24-year-old for having a child with Chanel and poppin' the question so fast, but it seems like things are workin' out perfectly!