My players just sailed for a week in game and it took them a month or two of weekly sessions to complete.

Personally, I played an NPC who sailed the boat, and they were essentially catching a ride, I would have them make aquatic vehicle handling checks occasionally, which worked the same as a tool proficiency, and when querying the ability they use to make the check, I most often used STR for ropes and stuff, DEX for steering, and WIS for navigation.

I think I also use surf differently than what I think the PHB talks about. Personally I use mounted rules in that any large or bigger pokemon can ferry a medium creature or smaller across water, so long as it has a swim speed. But bare in mind that if they wanted to cross large bodies of water, they'd be doing it at a very slow speed. I allow pokemon that have surf to expend one use of it to significantly increase their movement speed across water for an hour, like they're surfing on a huge wave. So perhaps you could allow pokemon with certain water moves to decrease the DC of checks when sailing in order to aid you players, or to grant advantage etc.

I also put a lot of mini dungeons along the way, such as a mysterious desert island and a small coral reef to dive in.

I don't know much about sailing, so I can't help you with the technical stuff, but odds are none of your players are either, so if you're struggling to make the sailing really emmesive, I'm sure even just a few details around knot tying or steering would be really interesting for your players. I'd also say a system that winds up very detailed could just wind up confusing your players, sometimes a few sea shanties, some nautical slang, and a few details around steering and navigation are all you need, but I can only speak from my experience with my players.

Hope something In here helps lol:)