That isn't the real issue.
Germany had a surprisingly small number of operational submarines at the start of both world wars. Wartime production added more boats but Britain added ASW vessels and aircraft at a faster rate so Germany never caught up. Admiral Donitz didn't want to spread his outnumbered force even thinner by transferring a significant number of boats to the Med. Who can blame him? Let Italy handle submarine warfare in their own backyard.
As for Med and submarine warfare....
4. Deadly Mediterranean - The U-boat War in World War One (WWI) - Kaiserliche Marine - uboat.net
During WWI the Med was single most productive theater of operations for German submarines. Pola Flotilla submarines established records for tonnage sunk per boat which have never been broken.
U32.
.....106,034 tons sunk.
.....18,554 tons damaged.
.....1 prize ship. 1,115 tons.
.....1 warship sunk. 14,000 tons.
U33.
.....194,131 tons sunk.
.....36,452 tons damaged.
.....1 prize ship. 453 tons.
U34.
.....257,652 tons sunk.
.....14,208 tons damaged.
U35. Most successful submarine in history. A record that has stood for almost 100 years.
.....535,700 tons sunk.
.....36,439 tons damaged.
.....1 warship damaged. 450 tons.
U38.
.....292,445 tons sunk.
.....26,139 tons damaged.
.....3 prize ships. 3,550 tons.
.....1 warship sunk. 680 tons.
.....1 warship damaged. 10,850 tons.
U39.
.....405,035 tons sunk.
.....30,552 tons damaged.
.....1 prize ship. 798 tons.
.....1 warship sunk. 1,290 tons.