Am I missing something here, Rudi?
The river shuttle from the Riverside Marriott operates to and from the River City pier, next door to the Sheraton. Upon request, it picks up/drops off at the Shangri-La pier.
None of these piers is located within convenient walking distance to the BTS, the elevated railway. Of course, I don't like walking more than 100 yards in Bangkok's heat and grime ... is there a new pick-up/drop-off location that I am not aware of?
Obviously, the Riverside Marriott location is more convenient for sightseeing by river; but the Oriental, Shangri-la, Royal Orchid Sheraton are even more convenient.
But a visit to the river and the hiring of a longtail or similar can be accomplished easily by an air conditioned taxi ride down to one aforementioned hotels, all of which are sights in themselves.
As for shopping, restaurants and the quintessential Bangkok experience, the J.W. is the place with the Nana hotel next door not to be missed. The BTS runs right in front of the hotel!
For a sketch of the Nana :
http://www.geocities.com/posh-boy/chap_25.htm