Tools: Complete socket sets (1/4 to 1/2" drive, with an adaptor to 3/4" drive, and a couple of 3/4" drive sockets for specialized items). Include in that hex (allen keys) sockets. Two large vise grips, needle nose vise grips, small regular vise grips, chain vise grips and strap wrench. Pry bar, alignment punches, Water pump, regular and several needle noise pliers. full set of screwdrivers, Jewlers, including narrow space right angle ratchet drivers, 1/2" blade large screw driver. Various other wrenches-such as spring clips. Hack saw, with extra blades, set of files--both rasps, rat tails, mill bastard, etc. Needle files. Pieces of solid wire in various sizes. Set of allen keys, set of box wrenches, cresent wrenches. Ball peen hammer, battery operated Dremmel tool (with drill bits and multiple grinding, cutting etc blades). Calipers, machinist rule, hand drill and bits (If I am going offshore, I put in battery operated drill and grinder), impact driver set.
We carry plumbers strapping, hose clamps, small amounts of bar stock, threaded rod in 1/4", 3/8" and 1/2", with nuts. An assortment of nuts bolts and screws, clips, ties etc. Also some appropiate short lengths of hose, hose barbs etc. All of these extras are in a medium sized tackle box. Extra filters, plugs, oil etc for each engine are also carried. We also carry several sheets of assorted gasket material. There is epoxy, both sticks, JB weld, and 5 minute clear. Spray can of Liquid rollers, wd 40, Aero Kroil, anti corrosion spray and gel. Also various sizes of electrical wire and multiple crimps, ratchet crimpers, small propane torch, with regular and silver solder. Assorted light bulbs, a couple of extra sockets and plugs.
We also carry a small hatchet/axe and folding shovel. Plus a cheater bar. Sure this sounds like a lot of stuff, but it is amazing what you can repair/build by hand. On our offshore boats we had a lot more, including a welder, drill press, considerable amount of metal stock, fiberglass and epoxy, engine gasket sets, spare injectors. One has to remember that the tow boat services only cover a very small part of the world.
Even on Lake Powell, almost every year we tow some boat back to a camp site or mechanic, because they have a break down or out of gas/bad filters etc.
One time we were some distance from civilization on our RIB in AK, and the fuel pump on the outboard started to fail. We carry an abreviated kit with us when we go in the dinghy, and found some "extra" hypalon on the inflatable, which we could fashion a new diaphragm for the engine fuel pump and got us home nicely--it would have been a very long paddle--in an area where these is not only no towing service--but communication is almost impossiable other than by HF or satellite radio.
I think if one always is in a place where a towing service serves and can be summoned, will arrive in a timely manor, and you have reliable communication that it would make sense. Once you go out of the boundries of these areas, then you will be on your own--even with the tow service.