Chitose-class seaplane tenders come with two Type A Ko-hyoteki as their default equipment at their level 12 remodel.
Seaplane Tenders can equip depth charges to boost their ASW. They are, however, unable to attack submarines unless equipped with a seaplane bomber. They can take down SS with the right equipment but the effect is fairly low.
Chitose-class, Mizuho-class and Nisshin-class seaplane tenders can be equipped with one or more Type A Ko-hyoteki midget subs to launch a preemptive torpedo strike. They can also do artillery spotting if equipped with main guns and seaplane.
Chitose-class seaplane tenders can be remodeled into light aircraft carriers at level 15 and lose all the specialties of seaplane tenders, but in turn, gain all the specialties of light aircraft carriers. You can find the stats of their CVL class at this page to aid in your decision.
It is advisable to keep both of the Chitose sisters in Seaplane Tender form until at least you have cleared expedition 36. Expedition 36 is needed to unlock expedition 37, 38, and 39 as expedition 37 and 38 have a very high resources drop per hour.