Hi! 

Maybe this is a silly question, but buying the "game" here,  will I have access to future updates, or just to 1.5 version? 

yes it gets updated here as well

Just go to the download link in the itch.io email you received. That is the link to your personal download page. Future updates will be shown with a version number and time last  updated.

Thank you !!

I downloaded from the page the email had sent me. The file size is smaller than the 1.5 and the 2 updates I had downloaded since.

In Explorer (Win10) it says the file is 3,540,503 kb / 3.37 gb. Is this the right file for the update with the ocean?

it should be

file size differences are optimizations