There are otters on Orkney, but they are not sea-going creatures and it is definitely sea between Orkney and the Shetlands. The Pentland Firth is quite definitely sea too, but considerably shorter/narrower. It would have to have been a fine day....

Just for interest, nowadays Orkney is preferred to the Orkneys, despite some historical justification for the plural. You do have to be careful with the term "mainland", as that's the name of the largest island in both archipelagoes.