In coastal waters you want to be very quiet speed is not nessasery in shallows because eventualy you will cavitate.
Submarines like the kilo german type 205 upwards are all coastal boats, they are not designed primarily for open ocean deployment but can do if nessasery.
Coastal boats dont realy have a top speed of any higher than 20 knots its simply not needed in shallows or littoral waters.
A nuke on the other hand is built half and half mainly for speed and also when it needs it stealth.
Anuke has more advantages it can stay out to sea longer go faster and carry more weapons.
The nukes can be transformed into verious roles take the 688i, its not only a hunter killer using torpedos to sink enamy submarines, its also a SSGN carrying anti ship missiles, while maybe at the same time carrying TLAM missiles for co-ordinated strikes far inland.
So its a heavily veried role, nukes need the speed conventionals dont, infact conventionals are far quieter than nukes.
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