Hi,
October is the start of the seal's breeding season. I understand that they become very protective of their area at this time of the year for obvious reasons.
Used to Sea Kayak, where you're sitting, floating with only a layer of plastic between you and the sea in a 16.5ft long, 22inch wide 'boat'. I have had seals bang the underneath with their flippers in curiosity but usually they just swim in your wake popping up now and again. It can be a bit disconcerting when alongside up pops a seals head, about the size of a football, seems to grin at you and then plops under again . However, they never did me any harm.
My sympathies to the dog's owner.