Unfortunately, there is no way to get back into any of the lower sections of Sentinel Island. In the original release of D2:ED I tried maxing my vitality and healing rate (with charms and enchantments) to see if I could last long enough to swim to the hidden cave, but still died by the time I hit the water.
Did you finish the Ancient Mariner quest? Reading the Ancient Journal yourself gives 2 skill points.
For D2:ED -
The official "ULTIMATE MAGE" guideDivinity 2: Ego Draconis All-Purpose GuideSince you have the PC version of D2:ED (from one of your earlier posts) see
this topic about possibly upgrading to D2:DC.