(I"m no expert, but I know a little - I was hoping someone better read would opine, but I'll take a stab, and maybe they'll do us the favour of correcting any errours I make...)
Actually, it's even more more [strike]silly[/strike] specialised than that...
If you look at medieval occupations, they are incredibly specialized. One group might cut the grass for twine, another would create the twine, another would sell the twine, another would turn the twine into rope, and yet another would sell the rope - and no one saw this as inefficient or contrary to common sense. But if you're going to do something back then, you do learn to do it well. Those who made barrels (coopers) did not make buckets, and vice versa, even if the construction only differs in scale. The reason why (before RPG's, ahem) the phrase "a Jack of all trades" was always followed by "and a master of none."
The more "agrarian" occupations did not tend to form Guilds - farmers, herders, gatherers etc. They were spread out, and at the bottom of the economic food chain. If someone had no other Trade, the best could aspire to was something like that.
Manufacturers taught the secrets of their trade to others via apprenticeship (and only they could then legally practice that trade) and that was the basis for one sort of Guild, and others sold that product and formed a Guild to protect their monopoly on that product and the prices (and profits), and that was the basis for another. The former guaranteed that outsiders could not legally compete, and the latter guaranteed that an outside entrepreneur could not simply buy the product and undersell them. (It also protected consumers from shoddy wares and services, and so The Law bought into the system and eagerly supported it - unless the local Lord decided otherwise.) Either way, the goal was to have absolute control over who can enter the market, and so from a Capitalistic point of view there is never any "competition" allowed - the Guild has a pure and total monopoly, and controls the size of the labor pool. (This is fine by those in power, and in part explains why there was little economic or geographic mobility in the social classes - you do what your father did and where he did it, because that's all the options available.) This monopoly was the goal, and it was achieved efficiently.
C&G lists watermen as a "Merchant guild" because they list all "carriers" in that Chapter - possibly because of the obvious capital investment needed - a boat (or a mule or wagon for land carriers. The "merchant" part was the contracting to carry.) As opposed to services, which may need little or no tools, that boat/mule/wagon/cart whatever is a huge signifier for those in that profession. Very difficult for an outsider to "blend in" with the locals. (Also, the route itself - everyone passes each other regularly, so will know each other by face). This creates a different dynamic than for services, where individuals can travel and roam to find work.
So, at the simplest level, the diff between these Guilds was the difrerences in the practical challenge of maintaining and enforcing that monopoly within their chosen market. If extended "families" allowed the sort of broader geographic connections that local commonfolk could not achieve, then that's how that challenge was (most effectively) surmounted.
