Anyone used this combo before ?
Sky swim lets you use a swim speed through air. combined with wildshaping beast shape 4 you can get your giant sea creature on and be e.g. a Huge flying squid or perhaps a giant gar and be a flying swallow whole machine.
any advice on this or similar things you could do ?
It looks like you would be limited to any creature that can still breathe air, or at least as long as you can hold your breath in that form.
If you can find a way to breathe air in an aquatic animal form, this would be a lot of fun to pull off, but otherwise the majority of air breathers I can find are either too small to be of much use in this fashion, or are too large to wildshape into.
Although paizo's polymorph effect give you the ability to breathe in water or earth if you gain a swim or burrow speed it does not say you loose the ability to breathe air.
remember its not like 3.5e where you were actually becoming the animal your just taking on its form and some abilities.
otherwise you would not be able to breathe air as an earth elemental ;)