Or HaChaim on Exodus 15:8/אור החיים על שמות ט״ו:ח׳

1 נערמו מים, גצבו כמו נר, "the waters were piled up, stood upright like a wall." This verse describes three distinct activities performed by G'd when He split the sea. 1) The waters split and remained in one place as a heap so as to make a path available for the Israelites to cross. 2) The waters which would normally replace the newly created vacuum in the centre of the sea did not spill into that vacuum but piled up along the sides forming a veritable wall. 3) As already mentioned in my comments on 14,21 it was not the entire depth of the sea which was split, but the deeper layers of water were frozen so as to enable the Israelites to march through relatively high ground and not to have to descend to what had been the bottom of the sea. The expression קפאו, "were frozen already," reflects what we have said earlier that the freeze of the lower waters in the sea preceded the splitting of the upper levels of the sea. The expression לב ים refers to the lower level of the sea which froze. The reason that the Israelites again used the word תהומות in the plural was that G'd made them travel along twelve separate lanes, each tribe having a lane to itself as we know from Shemot Rabbah 24,1.