We heard that there are good boat tours from downtown London to Greenwich. What are the best options? Do any have narration along the way? We will be there in early April
https://www.rmg.co.uk/plan-your-visit/visit-greenwich/river-cruises
The proper description would be central London not downtown.
I did a narrated river boat cruise from Westminster Pier (just downstream of Westminster Bridge) and thought it was good. The cruise lasts about an hour, but you don't have to go all the way to Greenwich if you don't want to. The most interesting parts are between Westminster and Tower Bridge.
At Greenwich, you are close to the Cutty Sark, National Maritime Museum *free to enter) and the Observatory. Yu can get the Docklands Light Railway (DLR, which is run by Transport for London and uses the same ticketing etc) back to the City of London and from there tubes or buses to almost all places.
If you want to get the Skyride Cable car back to the north side - it's over a mile away (from memory), so you can walk, get a bus or get a Uber river craft one stop.