There are multiple Seal Watching Boat operators at the Harbour in Hout Bay, they usuall go every 30 to 45min so just see which one fits your schedule. Price wise they are all around 50-75 ZAR, if you go for the higher priced, its likely that your boat is less crowded, which we can recommend if you want to get good views of the seals.
The Drumbeat boat was good for us and we had a great 30 minutes at Seal island. The whole tour is only around 45-50minutes.
Coming back you get greeted by an overly cheesy music band, not sure why they instated this, it doesnt fir the experience at all, but overall, it was a nice experience!