Rail >> sea. Especially given that Russia doesn't have full sea control outside of Sevastopol, at least not enough so as to guarantee continuous cargo transport.
Or if they deploy their fleet regularly outside of the harbor... well, we saw how that last went when atmospheric conditions aligned against them.
My understanding is that the Dniepro to Crimea water canal was one of the reasons for the invasion. After Ukraine cut off water supply after Russia invaded Crimea (you could see the canal dried up on Google Maps satellite), they were having to spend $$$ to ensure the peninsula had enough fresh water.
You can see where it sources from the river too: https://www.google.com/maps/place/46.7177240,33.4085303
Which should already be in Ukrainian artillery range from the opposite bank, but I imagine it's hard to stop a gravity fed canal from flowing with artillery...