You don't need to book in advance or buy a ticket if you
use the Oyster card (or a contactless credit card) to ride the Thames Clipper/Uber boats.
Just walk up at the pier you're starting from, tap the Oyster (or a contactless credit card) as you board the boat, find a seat.
We have always purchased an Oyster card when arriving in London or "topped up" one we had from previous visits.
Tap the Oyster card on the pad when you board the Uber/Thames Clipper boat. Tap out on the pad when you exit the boat.
Oyster card is also used on the tube (underground)--tap in when you enter through the turnstiles, tap out when you exit.
Tap in when you board one of the London buses, do not tap out when you exit the bus.
The buses are a good way to get around London.
There's no need to purchase a Hop On Hop Off ticket (or any other ticket) for the boat if you use an Oyster card (or contactless credit card) to tap in, tap out.
For a day trip to Greenwich using the Thames clipper, do you think it's smart to get the Hop on Hop off version just to open up the possibilities on the trip back?
No. You don't need that if you have an Oyster card.
You can get on the Thames Clipper anywhere you want....and get off at any location you want. Then you can get on the boat and do it again....going onward to another location.
Oyster cards can be purchased at machines (looks like an ATM machine) at Heathrow, or at the entrance to tube stations, or at some convenience stores and grocery stores. Or at the London Visitors Center near St. Paul's.
Using Oyster for the journey to Greenwich has always worked well for us. I have not used the Hop-On-Hop-Off ticket on the Thames Clipper/Uber boat, so maybe someone will come along in a minute and compare the costs, etc.
We most often leave from the Tower of London pier to go to Greenwich. It is...well...right behind the Tower of London. Convenient if you want to see the Tower as soon as it opens in the morning, then take the boat to Greenwich after that.