Here is the data sheet for that part.
https://media.digikey.com/pdf/Data%20Sheets/Airpax%20PDFs/APG,UPG,IPG_Series.pdf
The manufacturer suggests the IAG series breaker as a replacement for this obsolete part.
Here is the data sheet for the IAG series.
https://www.sensata.com/sites/default/files/media/documents/2018-04-24/iag.pdf
Why is it a three pole breaker? Can the boat be properly and safely rewired to use a two pole breaker?
Here is a 30A two pole breaker.
https://www.digikey.com/en/products/detail/sensata-airpax/IAG66-1-61-30-0-Q-01/5082808?s=N4IgjCBcoLQBxVAYygMwIYBsDOBTANCAPZQDaIA7AEwDMIAugL6OFVkgCSAggOIBsfGGBh9hNAAwA6cTACKMcRCZA