Out of the three treasure chests, there are two that you can access immediately.
Of those two, one () is the relatively useless Clothes.
When you're done examining the treasures, drop down any of the pits to continue your journey through the cavern.
By falling through the pits in the floor above, you will land in the chamber at the top of this floor.
After you ditch your boat at the dock north of Crescent Lake, slip your canoe into the river and use the map above to find the route to the Cavern of Ice.
Just as you attacked Fire monsters with Ice spells and weapons, Ice monsters are susceptible to attack by fire.
Monsters will not engage you in battle while you are moving across the ice, so tread the areas to avoid confrontations.
Fire is your friend against the ice-wielding enemies in the cavern.
To make matters worse, the Clothes are defended by powerful Dark Wizards (Mages), who have the ability to cast Kill, and instantly wipe out one of your party members. The third chest, , is completely surrounded by pits.
You'll have to locate another means of reaching it.