ApovA idagam sarvam vishvA bhutAnyApaH prANA vA ApaH pashava ApO$nnamApO$mritamApas-samrADaapO virADApas-svarADApash-chandAgsyaapO jOtIgshyApo yajoogshyApas-satyamApas-sarvA-dEvatApO bhoorbhuvassuvarApa Om!
"Aap" seems to imply To obtain/attain/get, To reach/go/overtake, pervade, occupy; To become filled; To cause one to feel or perceive; Water, one of the eight Vasus; Sky etc. In the context of the above verse it appears that everything is water, but on the other look each time Aapa is used it seems to infer a different meaning to the word depending on the associated idea. For example: "Aapova idagam sarvam" can be "that everything that we see around is nothing but water or sky; or that water or sky pervades everything"! But, in the context of "prANa vA aPaH" the prANa is all pervading, and the this vishva is pervaded by sky and the "pashavah". That that all pervading thing is "satyam"; the devatas are to be obtained or reached in "sarva-dEvatApo" and so on. I am not sure because this is just a superficial look at the idea.