A slide may be a useful buoyancy aid near the shore, but with the sea being twice the size of the land, it has some pretty remote areas. A liner will reach maybe 25kt, so you'd best be within 100 miles of one to be much use at all. Food parcel drops are possibly out of range of helicopters and anyway, hope you have a load of paddles to hand or you'll stay floating at a distance.
Just when you thought things on a slide raft were pretty miserable, though, with under 3 miles visibility of you can even stand up in perfect weather, and no means of signaling apart from a Christmas cracker whistle and a small, transient reading torch, you actually have to load balance accurately immediately. An overloaded raft is useless, a 44 person raft will fail at around 55 people, so where you have 400 people you have to avoid failover cascades. A challenge by still, sunny day with calm people, probably not going to get much easier if the weather is in any way cheeky.
Personally, with the inner engineer actually looking at the resources and facts, I'd say the slide rafts might double the survival rate over the 1-4 hours term, but your overall survival chances probably go from approaching nil to approaching approaching nil. The safety equipment can help in certain limited circumstances, but I reckon a lot is for show and reassurance, in many practical cases.