The minefields are almost everywhere on that area but the mines are under the surface. So if you go to periscope depth and further down you run the risk to hit them. If you're on the surface you don't need to worry about mines, but you need to be careful about DDs and planes. That's the reason why I'm suggesting to run decks awash. If you are attacked you can dive quickly, and you can minimise the chances of being spotted by DDs.
When I did it I was on the move all the time. I was running decks awash and I was trying to set up ambushes to lone merchants. At night I was attacking from the surface and I was finishing up the target with the deck gun. During day time the attacks were done at periscope depth but with all engines stop (that way you avoid being hit by a mine). The conning tower was a bit exposed but if you raise the scope just a bit you can still see and fire a shot. Don't forget the merchants and the DDs are not too good in spotting you early in the war, so you can use it to your advantage and be a bit more aggressive. Because of your small profile you will most probably see the ships before they see you. If it's a merchant you can plan your attack without any problem. If it's a warship you're still running decks awash. You don't need to submerge straight away. Depending on the time of day and the weather you can always position your boat far away from the warship and still keep moving on the surface. If you're spotted or if you need to dive, then you have to pray you're not inside a minefield. If that's the case I would suggest you either go to the bottom and stay there until the danger passes or if you need to be moving try to do it at small speeds and for short periods of time.
As I said before it's tricky and dangerous but it can yield a lot of tonnage, and early in the war it's a unique oportunity.
Good Luck
(and trust me you will need it
)