Use the boat from Dread Dock in Reaper’s Coast to get to Bloodmoon Island
If you are in Fort Joy, progress to Reaper’s Coast first
From the Lady Vengeance, sail to Driftwood, then head to the Bloodmoon Island route
Use the waypoint near the Elven Camp in Reaper’s Coast and travel north toward the coast
Cross the water by boat or teleportation skills if available
Enter the island through the dock area on the eastern side
If you meant Bloodmyst Isle from another game, that location is not in DOS2