So I am not quite sure about this but with 1000+ hours I should be able to find a dig spot on any island.
Problem is that (in my opinion) the one digspot below the 'new' Ship being built on Shipwreck Bay is not accessable any more but can still appear in riddles.
Text is 'At the wrecked rowboat on the South-West shores ye are almost there, 10 paces North-East dig treasure bare'
Please check this...