That is correct, both of the partial loops are a long ways away from the marina. You could take the internal bus from the stop at the loop to the Settlement Depot which is where the marina (short walk) is located. Without getting in trouble for expressing my feelings as a camper about this let give a couple of gentle suggestions:
If you get a partial hookup site (the cheapest) there are 2 loops, the 1500 and the 2000. If you don't plan on going to the fort, go ahead and request the 2000, most campers prefer the 1500 and there are twice as many sites in the 2000 loop.
Better yet, if you want to aid the campers, spend the extra $10-15 and get a full hookup site, there are way many more full hookup than partial and you would be doing a favor to the tent campers by freeing up one the limited number of available parital sites.
Hope I'm not stepping on any toes by suggesting this.