Do not confuse the 2GM20 and the 2GM20-F, they are not the same. This kit for the Yanmar 3GM and 2GM-20 does not include a water pump pulley because on these engines the water pump gets a separate belt that goes inside the alternator belt. See photo below:
These pulley kits by Balmar offer the opportunity to install a larger alternator than would otherwise be possible; this kit is for the Yanmar 23GM and 2GM-20. Alternator upgrades using a single drive belt are limited by the amount of power that can be transferred by the belt but using a multi grooved belt considerably increases the possibilities. The kits include a set of serpentine pulleys for the crankshaft, water pump pulley and alternator. This pulley kit comes with two belts so you have a spare, the alternators need to be purchased separately. If you order an alternator at the same time as the kit it will come with the appropriate pulley pre-installed.