Or if you want to travel to far away places warm sunshine and tropical breezes... save your renoun for a XIB. You'll carry more torpedos and be able to sink more shipping. It's very nice for those large convoys you come across and want to wreek havoc on. The upgrade is in the eye of the beholder.
But putting the far away places aside, with SHIII Commander 2.3 installed you can select the partol area you want. Even the Uboat number you wish to have, along with other changes.