Buy bigger boats to sail to new fossil islands. That is the exploration progression stated by the developer; island-by-island prices and boat requirements are not confirmed here.
On this page
Choose the boat goal before leaving
If your next goal is another fossil island, boat progression is the named route. Check the current boat offer and destination before spending; this is a general purchase check, not a published boat-price table.
Separate travel from digging
Reaching an island and successfully using its dig sites are different tasks. The public description links bigger boats to access but does not establish shovel tiers or per-island fossil drops.
When an island remains inaccessible
Note the exact destination and message shown. A universal price, level gate or timed unlock cannot be inferred from the bigger-boat instruction alone.
Your next task
Choose the guide that matches the step you are working on next.