If you gain no more advice from this entire web site than you do from this page, you will do well.
Bring warm clothes, and plenty of them!
Caithness is renowned to deliver 4 seasons in one day - literally at any time of year. The wind howls over the flow country, and particularly when coming from the north or east can be bitterly cold.
There are few places on the river that require, aye that benefit, from wading - the exception being Loch Beg on 12 where it can be advantageous if you cannot get out on the boat. However, you will find that waders do keep you very warm and dry so I would recommend them at most times of the year.
If you decide not to bring waders, then ensure you bring warm, dry, comfortable footwear.
I'll repeat my earlier advice for emphasis - bring warm clothes, and plenty of them!!