Well saving to close to convoys and other ships has been know to cause Ctd's this goes right back to stock SH3. Way before GWX hit the scene.
As a rule of thumb you should never save your game when you are:
200km or less from land.
near/ in sight of convoys or ships that you have just sunk. ( ie you have got the shes going down message but she has not sunk.
when you are sumberged.
I really feel for you mate, I came across a convoy like you did it had been attacked by a wolfpack the large circle on the nav map. i sunk the whole convoy was up to 90,000t and had torps to spare. But a bit later i saved while i was to soon after a sinking a ship and i became a victim of the save game bug.
