Let's start with what you don't need. These are the tools you don't know how to use or functions they perform that you can't.
I have what can only be described as a very complete home workshop. There was a thread on this board several years ago in which posters declared that they had better/more tools on board than I had in my workshop! And they posted pictures. Strangely most of the pictures were snipped from websites, very few showed real-life backgrounds. One poster owned a small Tri-maran which I'm sure would have sunk if he really had the tools he said he did.
OK, so what do I do? When I bought my boat in the US and had a couple of weeks to prep her for the voyage back to NZ, there was not even one screwdriver aboard.
I went to Home Depot where I bought one of those black plastic "briefcase" tool sets for $100. To this I added two sets of open box spanners, a screwdriver set, a hacksaw, a battery drill with some drill bits, a Stilson wrench, some pliers, cutters and a vice grip, a 12v soldering iron, crimping pliers and some wire. There may have been some other bits I can't recall.
I sailed home with only those tools and honestly, it's a rare event when, even now ten years later, I have to take anything else aboard. Almost all of the jobs I do on the boat can be achieved with just them. If there is a reason to take tools from home, they always go back when the job is done.
So my advice is don't clutter your storage with tools you may use once in a blue moon. Rather use the space to store logical spare parts and materials that are hard to source away from main centres. Consider what you are likely to have to fix and tool up for that and no more.
And as with most things on this board, it depends . . . If you're day-sailing a 24ft boat around San Francisco Bay or if you're taking an aging 44ft across an ocean, your equipment requirements are likely to be a little different. If your diesel has 200 hours on it, don't carry tools needed to replace cylinder liners.
And don't be told that you need to be able to service/repair a space shuttle at sea, you don't.