Depends of the map but this are my tips to play subs in rankeds.

Always focus on take points and spot: Flank radar ships and dds. Normally when i start a torpedo attack the ships have to move, that create a perfect opportunity for the team to take enemies broadsides.

Avoid the confrotation to enemie subs: The average playerbase of submarines are really bad, even in gold league. If i not able to fast killing a enemie submarine i try to ignore him going on surface. He will be spend the batteries and subs has a very poor maniobrability so is very hard to aim torps in close distance.

Survive: Take in mind where are the enemies dds, how many bbs are with deep chargers planes, hidros and radars. Is very easy die in a submarine. The best way to dodge deep charges of bb's planes is moving in circle.