Saltmarsh is bascially just an anthology of martitime themed adventures anyway so not sure how you can not go off path with it but it is easy to add more options to it!
Here is my current "Saltmarsh" campaign. I started out with U1 then an idea of where I wanted the campaign to go..kind of. To start I had 3 possible paths/ rough ideas.
1. Follow from Saltmarsh to Smugglers and possibly link the the Slavelords series
2. Follow up lizardmen to U2 and then and possibly lead to Tomb of the Lizard King (eventually)- I had a group of lizardmen arrive at the mansion to meet the smugglers (the party didn't get to the Sea Ghost).
3. Follow up on some other threads (some Dungeon mag adventures) and lead to isle of the Abbey/ Evil Tide and other sahuagin adventures (maybe the whole 2e Sahuagin series).
As you can see at the end of each adventure the party were usually given (or had already a good hook or two to follow up if they wanted) their next adventure choices. Only Light of lost Souls was directed as the fighter had died in U1 and his uncle being the inheritor of the mansion and their sponsor they were asked to clear to start the series wanted his nephew and heir back so they owed him and the church a little favour...
Post Evil Tide they wanted the hell away from the sea and sahuagin so I threw in a little bugbear raiders homebrew while I sorted out where to next.
I've tried to maintain 2-3 options for the next adventure for them to choose and that has lead from the Sea to the heart of the crystalmist mountains
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Red arrows indicate choices not taken or the next options to follow up.
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There has been minimal prep from me but the players are directing where they go rather than having little input at the next "adventure decision point" (which really isn't) or chapter in the adventure path. I much prefer this style game to a more linear structured adventure path. Sure you can, and should add material to adventure paths (or take away form them too) but this style of gaming is much more my preference for D&D.
It was interesting early on the arguments between different options between the players as they valued different choices (especially as they wre keen to deal with the smugglers but had a few weeks before the smugglers were returning to Saltmarsh so wanted something todo till the Sea Ghost returned on the enxt full moon. I find that after 3-4 adventures the next step organically grows out of previous choices and once the players become invested and feel their choices matter and are driving their story the game grows organically and gathers its own momentum more so that we need to do A-->B--C to "finish" the adventure path.
I could start this campaign again with U1 and another group and I'm sure the players would choose a very different path as they went.
Stormdale