Most outboard ignition switches connect as follows:

B---positive battery--usually at start solenoid-normally protected with 20 amp fuse.
A---accesories-hot when switch is on
C---choke-hot when key is pushed in to choke position
S---starter-hot when switch is held over to start position
M and M---magneto-one M is connected to ground -black- and the other is connected to kill wire-black with yellow stripe-. When switch is off these two are shorted together.

Another good thing to remember is that a boat doesn't have a frame or body ground as does an auto or most household items.
With that in mind, you always have to route a battery ground as well as a battery positive when installing new accessories, lights, etc.
Most boats have a ground strip and fuse panel somewhere near the cockpit or dash to make this easier.
If yours does not have either, then you might want to consider picking up a marine ground strip/fuse block or fused switch panel.

When installing a sensitive electronic accessory such as depth sounder, VHF, GPS, or Radar it is always a good idea to have a separate ground all the way to battery to prevent interference.

(Helps cell phone static sometimes too!) If you are going to be using cell phones etc. at lighter I would run both wires to battery and use 10 gauge for hot and 12 gauge for ground. Going straight to battery and using a 20 amp fuse in line would cause less interference!

Slow Starter Solutions

You might want to check the solenoid and/or starter.
When you hold key in start position then 12 volts should be present at the big terminal on the solenoid with wire going to the starter.

If you have 12 volts there:

Most likely your problem is either in the starter motor: defective or worn brushes, bushings, armature, etc.... or..... not enough amps getting to the starter due to a problem in the negative or positive cables from the battery all the way to the starter.

It could be in a loose or corroded connection at battery, solenoid, starter terminal, or ground connection on engine block.
The cables themselves could be deteroiated inside as well!

Sometimes; but seldom a solenoid can cause an amp loss.

A quick substitution check for amp loss in old battery cables:

If you take a good heavy set of jumper cables and connect to your battery and carefully connect the ground to a good block ground near the starter; then carefully connect the positive cable to the post your positive battery cable goes to. Now try it to see if starter now has enough torque and rpms to engage drive and spin motor each time.

By trying the same method; substituting only the ground and trying it, and then trying only positive cable substitution you can determine which cable is the culprit.

If only one is bad, you might replace it alone, but if both are same age well, you know how it goes....