I'm kinda new to D&D. I can't understand on which adventures I can use the guide. If you could explain when and on what adventures am I allowed to use the guide it would be very appreciated.
[RPG] When is Sword Coast Adventurer’s Guide used?
Since this question was asked and answered, Wizards of the Coast has published Xanathar's Guide to Everything. Among the many additions and clarifications, pages 78-85 give a very detailed treatment on how to use all tool kits and what can be gained by proficiency with them.
It explains the difference between a skill proficiency and a tool proficiency, as well as how they can work together to accomplish specific tasks that would be more difficult or impossible without them. Each tool has a number of skill synergies which are used as an example and guide for the DM, as well as a table of common DCs for various tasks one might use them for. These are well thought out, listing activities that might actually have a use in play, such as using Alchemist's Supplies to produce a puff of thick smoke.
It is notable that the DCs listed don't necessarily require the tool itself, but that proficiency in them might allow a character to perform a task that would be normally impossible or more difficult for the unproficient character. For example, a character with proficiency in Brewer's supplies might allow one to detect poison in a drink
In addition, it lists the components for each tool, so that players and DMs can have an idea of the items actually at their disposal.
Like everything else in the Guide, the rules are optional (the text calls them "advice"), but they do provide a wealth of detail where before there were only the most general of guidelines.
Adjust your "course" to match their "straying".
A DM without players is an arbiter, not an adjudicator.
A DM with players is an adjudicator, not an arbiter, if they are a good DM.
It actually doesn't matter if they did or didn't follow your planned course. If you put an ocean there, expect that it might be traveled. I suggest that you jot a few notes down (not a full treatment, mind), just enough so that if they go that way, you have something to work with.
Those land obstacles aren't going anywhere, you will have an opportunity to recycle and reuse them. Perhaps you could even plant them in front of wherever they happen to land when they get tired of oceanic travel. Keep track of the passage of time, note how events will unfold without the presence of PCs over landward, if that is important to the events. Perhaps remind them from time to time. Or, you could take the land quest and make it an undersea-land quest.
The best way to improvise, is to plan ahead. Be flexible. Make Murphy your best friend, expect the unexpected, and then realize that they will probably do something not covered by either. And never, ever think, "well, that destroys my plans". Ever.
Instead, think, "how can I use this twist to my advantage...."
From the back of the SCAG:
You do not need it necessarily, but it may be useful for homebrew campaigns and if you want to use the provided character options.
The official adventure/campaign books provide enough content themselves, but if you want to expand the world the SCAG may help you with that as mentioned in Strom King's Thunder: