Many of the cards in my version of Mine a Million are open to a bit of interpretation, but I think this one is fairly clear.
- This card specifically mentions "warehouse stocks". It does not mention barges or ships.
- Similar cards for losses at the pithead specify "on the ground (not in lorry/barge)" and/or "in lorry/barge" (wording not exact, I couldn't quickly find the wording online and I'm not at home to check).
On the basis that the cards make it very clear when stocks in a barge at the pithead are at risk, I suggest the interpretation that stocks in a barge or ship are safe against earthquakes.
In fact, just last night when that card was showing, I put all my stocks (from both my players) in a ship and left my wife's stocks (from both her players) sitting in the warehouse. Unfortunately she managed to sell them before anyone rolled a 1 or 6 :(