Typically you'd choose wire that is adequately sized to have less than a 3% voltage drop in the run you are using. Let's say a cabin light in the v-berth, 30' away from the breaker panel including all the zigzags in the line. That makes it a 60' run, because you count both sides of the circuit. With a 20-watt bulb in that light...you'd draw less than two amps, so you could use a thin wire pair that only has a 3% voltage drop at 2 amps, and it would be good enough to give you a nice bright light. (In practice, you'd probably use a heavier wire and "t" off multiple lights all the way along one side of the boat, doing some rough figuring for the total of them all and something less than the total length for something that complex. In practice, you might just pick up the closest spool of wire you had, since 100' spools are cheaper and you just might have 14g at hand.)
The actual power handling capacity of the wire, called ampacity, will be well above the "3% voltage drop" size. Ampacity depends on what the insulation type is, and whether the wire will be in "free air" or buried in a bundle or conduit, which traps heat. If you exceed the ampacity rating--the wire catches fire, which is not a good thing. If you pick wire which has more than a 3% voltage drop, you just choke the devices at the far end.
For instance, if you use 10g wire to feed your masthead light, the light will be nice and bright. But if you use 14g or 16g wire, you can save some weight aloft. To a diehard racer...the 14g wire and saving weight aloft might be the right answer. If you don't want to get run down at night...10g might be better.
Sources for crimping tools? I'd avoid no-name junk from China because it is easy to make the dies the wrong size. Any brand name from any reputable source should do. Palladin is one of the "second-tier" brands you'll find at a lot of computer stores, etc., that seems to be quite good for casual use. (Might not hold up to 50,000 crimps in industrial use but that's not us, right?)
Something else about crimps that I don't think anyone has mentioned here. The fittings should be tubular, or lined with tubular liners (copper or tinned sleeves) and not split metal. The $5/100 kind in the auto stores are always split metal, not seamless tubing, and if you crimp the tooth down on the split side--instead of the seamless side--the crimp will never have full strength. There are real differences in "the good stuff" that way.
You'll see most ratcheting crimpers have a removable/replaceable crimp die. If you find one that doesn't, don't worry about that. In practice you don't swap dies, you swap tools. You remove the die to replace it after it has worn out, which shouldn't be an issue for "home" users. Looking at how clean the tooling is, the overall quality of it, is more important. If you need to crimp TV cables or something else, buy a second crimper not a second die.