We had planned to take the Thames Clipper from Westminster bridge to Greenwich pier but decided not to as we could not see how to get our luggage down to the Westminster pier other than dragging down lots of stairs. Plus it is very crowded n London now. Took Uber to Greenwich pier (only 20 GBP) . We had to look around for drop off point because could not find the tent (not there now). Eventually found some Viking folks.
I think if you Google Greenwich cruise ship tier it leads you there. We were early, 10:40 but found someone to take our bags. They were still unloading people and bags from prior cruise so best not to be too early. At 11 Viking opened the check-in building and check-in went smoothly.