I see that the CC water taxi now has a website where you can buy your tickets in advance. Previously, we would just buy them at the depot when we got there. Is there any advantage to buying them ahead of time? Will it hold space like a reservation? Since there will be 6 of us coming, catching the last taxi, should this be a concern?

I've never seen them leave anyone behind, even if it means overloading the boat.
It's every man for himself at boarding time, get in line early with or without a pre-paid ticket if you want a reasonable seat on a crowded day!