Unfamiliar with 777, so only in reference to 737:
On 737s I presume there are just slides on the wing
No slides on 737 over wing, all 737 slides are classed as slides not rafts, as per STBYRUD's post. Operators flying beyond 60 minutes from land are thus required to have portable rafts installed or carried in cabin, usually found in overhead storage compartment in centre of the isle.
I have seen a FCTM for the 737-200 where it is advised not to use the rear exit doors in a ditching because of the tendency that that area will be under water. That paragraph not in the 737 Classics or NG series as far as I know
Still applies to all 737 models, due to fuselage shape not enough buoyancy in aft to keep it above water within normal range of CG.