A mile and a half away, moderate to heavy seas.......tough, not impossible, but you would be a tough target.
Naval gunner, especially back then, had several different ways to get their rounds on you (yes, even civilian merchants).
1) O.S.O.L.O.O. Stands for One Short, One Long, One On. Meaning where the rounds land. One short of you, increase the angle of the barrel, one over you, decrease the angle of the barrel. you keep doing this until you find the right angle of the barrel that will make a balistic tajectory to place rounds right on you.
2) Walk it to the target. This has the side benifit (for the shooter of course, not you) of hurting the target psychologicly. There's something about seeing HE rounds slowly, but surely, coming closer, and closer to your target. The gunner will see the hit instead of a splash and know that they have the right angle on the barrel of their gun.
3) Spotting. There's another crew member with a set of binoculars. They are watching where the rounds land, and saying that information to the gun crew. This allows the gun crew to adjust the angle of the barrel, and train it around in bearing so that they can put a bead on you.
Basic rule of thumb: When you start getting shot at, it's time to dive, and dive quickly! Ships merely have to float, you on the other hand need to float and dive. A ships hull can take a pounding and still stay afloat. Your hull on the other hand should be treated like an egg shell! One or two hits from a small caliber weapon can place your sub in serious danger the next time you go to dive.
Be thankful that these are not modern day HE rounds. They can actually be set with a timer to go off. An air burst of one of these simply NEAR your sub, and it would be hit with shrapnel!
Oh, and by the way: Merchant gunners were trained! Here's a link that talks about US Merchants:
http://www.usmm.net/gunnery.html#anchor266245