We are planning to go to Wellington via ferry
Would taking a boat cruise in the sound be worthwhile is redundant?
It depends on how much time you have. The Sounds are quite extensive and the ferry sails right down the middle - first in Queen Charlotte Sound & then Tory Channel . You don't see a lot of the houses etc along the shoreline.
If you have a free day, it might be worth taking a water taxi out to Ships Cove & hiking to Resolution; or you could take a water taxi to one of the resorts for lunch eg Furneaux Lodge. The water taxis drop off groceries & pick up people from small bays and it is an interesting trip.
If you have a full day it could be worthwhile. There are two main sounds, Queen Charlotte and Pelorus, and the Ferry only sails through part of Queen Charlotte Sound.
If you have time and the means to get to Havelock, you could do a cruise in Pelorus Sound which would give you the opportunity to see a totally different part of the Sounds.
Havelock is over an hour drive from Picton, either via the narrow and winding Queen Charlotte Drive, or south from Picton and along Rapaura Road.
Both Queen Charlotte Sound (From Picton) and Pelorus Sound (From Havelock) have mail boats that service remote parts of the respective sounds, and offer passenger cruises.
Ok this sounds interesting
For a day hike and exploration which is better this track or Abel Tasman ?
They are different. Abel Tasman is a National Park and has lovely beaches. If you do a day hike in summer,, allow time for a swim along the way. OTH because it is in a National Park there are very limited commercial premises eg to buy a coffee or an ice cream (just Totaranui & Awaroa Lodge iirc)
The Queen Charlotte Track passes through private land as well as conservation land. A number of the bays have small "resorts" where you can buy lunch/beer/coffee etc - eg Punga Cove, Furneaux Lodge etc. Being quite steep sided, QCS doesn't have many nice beaches.
I have done the boat cruise twice, the 2nd time with overseas friends. I have travelled on the interIsland ferry many times. They are very different experiences. Although I took my friends to Wellington the following day on the Inter-Islander and they did not mind. You see much more of the Sound plus have a much more up close experience on the boat cruise. However it does depend on the amount of time you have.