I have acquired frost walker boots and gone to several deep oceanareas ( with ocean monuments to be sure) and I've walked around for about 20 minutes and the trophy is still not popping up. I've tried restarting the world and have tried several different times and I still can't get the trophy. I've looked up walkthroughs and I'm doing everything right. All my other trophies have been working just fine so it seems to just be  the this one for some reason