I had an old pc, I tried the game on that and had similar problems...it turned out to be the video card. Have you tried starting outside port and sail out to where no land or other ships are before going to the bridge? I did that and noticed when I went to the bridge my game wouldn't crash but my Boat was "invisible". Even the site of a seagull would crash the game.
Also, I may be mistaken, but I think there's a 4GB download mod here on the forums.
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