well its longer and more expensive than the train.... but a heck of a lot nicer.
Tube cannot do from where i am, DLR will be near my flat in 2009 but that goes the wrong way really.
Via train i can walk to the station (10 mins) then 35 mins on a packed non-aircon train...
Where as the boat is air-con, always get a seat, and get fab views.
It winds its way though the Thames Barrier, around the o2 Arena (the dome) and various other sights, it really is a nice way in. Boat is fast at pulling away too.
Train is about £120 ish for a montly season. (Zone 4 Oyster)
Boat is £152 takes 50 mins, but its only a 5 min walk to the dock, a monthly 'season' is a 40 trip pass, 1 way = 1 trip. There is a 3 months unlimited season which works out at £135 ish per month, i will take that as soon as the new boats come into service. Only bad thing its not zone seaon so only works on the boat, but i don't use the tube anyway...
Currently its commuter hours only to my stop, but soon its going to run all day and all weekend.