how far is it to walk from the cruise ship terminal to see the marina sands observation deck?. or to walk to centre ?
Which cruise centre? There are two.
From Marina Bay Cruise Terminal, Marina Bay Sands is about 2.5 km (~ 30 minutes walk). I don't think it is an interesting walking route so its best to take a cab. There are better walking routes around Marina Bay which you can start at Marina Bay Sands.
The other cruise terminal (Harbour Front) is not within normal walkable distance from Marina Bay Sands.