I with you for tying on. If slings with water knots will hold a factor 2 on rock they'll sure hold you on a boat. The loads are much less. The store bought tethers are just convenient. Watch out using climbing gear in a salty environment, they corrode really fast. Aluminum biners with steel hinge pins are an invitation to go over the side.
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217