My girlfried of the time and I had been away a few times on holiday but it was all city breaks. The only one beach holiday we'd been on was with friends so there was no chance of her going topless there.
We went to the Caribbean on our own in the mid 00s. She'd always been reasonably shy but a few days in to the holiday I was putting sun tan lotion on her back and moved her bikini top straps down to do her shoulders. She undid her clasp so I could do her back too. It was fairly quiet at the pool we were at.
I thought she'd hold on to her top at the front but she let it fall down exposing her breasts. It was such a turn on.
It obviously got her going too as when we went in to the pool a few minutes later we were cuddling at a semi-hidden spot and she took my hand and pulled it between her legs and before I knew it, I'd slipped a finger in her.
I actually sneaked a picture of her while I was over at the bar getting a drink and took a couple of others when she was on the sun lounger and she's got no idea they exist to this day. The quality of this one isn't great due to the camera phones of the day but the others are clearer.
We split up a few months later but I'll always have those memories.