I have the exact same issue here.
First time playing any subsim game, now on my first career 5th patrol.
This is the first time I encounter this MessageNet.dll crash.
For full story:
Start off from pearl harbor, head to midway asap for a refit, then head west.
About 350 nm west of Midway I come across a glitched task force. This task force has 3 battleship and 1 carrier plus some escort. If I shot torpedo at the leading battleship, the game crash as the torpedo get close to the battleship. I can avoid this crash by alerting the fleet by any other mean first before attempting a torpedo attack at the leading battleship. Tested method include sank one of the escort with deck gun, torpedo other capital ship, surface right by the lead battleship. In the end I finished the task force's all 4 capital ship, and the rest of the escorts just move along. On my way back to Midway for a quick refit, I found another task force with 4 maya. Killed one of them.
After refit at Midway, I go west to Honshu. There I make two saves in a roll (no ship around/ on surface, TC1) and then tested the saves. The saves works but upon loading the saves I get "torpedo in the water" message like 16 times. Since the saves appear to be working I decided to call it a day. Next morning I wake up and load the saves, the MessageNet.dll crash happened.
Applied the 4 gb fix and then start off from the commander office again for the same 5th patrol. I encounter the same task force and the same torpedo attack crash has occurred. Not sure what to do next.
I guess I will just follow the same route again. Hopefully with all these fixing and patching the glitches will be gone. In any case I won't be leaving the game until I finish this troublesome 5th patrol. I will still be making saves so I can test them out afterward.
If anyone here would like to investigate these crashes I can provide them with my save files.
Spec:
Windows 7 x64.
16 gb ram, i7 6700k.
970GT
DX9.0c and latest display card driver installed.
Stock v1.5 without any mod, fresh install in c:\game\