I have two small plastic tool boxes aboard, and 2 watertight containers with odds and ends in them.
Toolbox #1 is one of those cheap flat sets from Husky with molded spots for all of the tools it contains. It has basic 1/4", 3/8"/ and 1/2" ratchets, sockets and drives. Some pliers, adjustable wrench, small vise grip, allen keys, and basic screwdrivers. I got it 7 or 8 years ago on sale around Father's day at Home Depot for about $69 or & $79. Just before Father's Day seems like the time these things are around the most, and for decent prices.
Toolbox #2 Has other loose hand tools that I've found come in handy over the years, including a tape measure, assorted kinds of tape, cheap multi meter, and 12 volt test light. I've stocked it as the years have gone by with things I have more than one (sometimes more than a couple) of at home.
One of the watertight containers has various 12 volt electrical connectors, fuses, couple of feet of wire, and other electrical odds and ends.
The other watertight container has various pieces of stainless steel hose clamps, screws, nuts, bolts, washers, etc., that I've accumulated over the years.
A couple of battery operated LED flashlights, and 12 volt lights round out the onboard collection.
It sounds like a lot of stuff, but it really is fairly compact when packed away.
If I know I'm going to the boat to do maintenance or a repair, I bring down any tools that I think I may need from home.
The stuff I keep aboard is for emergency repairs, and for the times when I get restless while hanging out at the dock and decide to putter around.
Of all the things I keep aboard, I think the thing I use the most is the 12 volt test light and the watertight container with the 12 volt electric stuff. Most times when something stops working, or otherwise acts up, it turns out to be an old weathered connection causing the problem. A few minutes of diagnoses, and a fresh connection usually solves things.