I have to say Great Yarmouth doesn't have a lot to recommend it, and I felt bad cos I was being shown around by someone who lives there (this was a few days ago). The pier and the town had the air of somewhere struggling desperately. It was sad and windswept. The beach and seafront are huge, empty and bleak, huge empty vistas. Many buildings have seen better days, and the shops all look like they're about to go under. Bleak times. That was the impression I got anyway. Maybe I missed the nicer bits.
My favourite seaside town in Penzance. It feels like a real place, with real people living and working there, not all tourist stuff. The harbour has fishing boats, and the ferry to Scilly (and is the best place to eat fish and chips). The town is pleasingly hilly, with narrow backstreets full of character, lovely old buildings, and a high street that isn't just chain stores. It has nice botanical gardens. Views of St Michael's Mount across the bay. Pirate-ey pubs. Lovely helpful GP surgery
(I got a poorly thumb whilst there). Good sailing (did a week there in a sailing comp many moons ago). Nice houses. There seems to be interesting things going on, arty stuff, gigs, local bands, some political stuff I noticed. It doesn't seem completely second home territory.
2nd favourite, Whitby. Harbour again, a working place. Abbey, hilly too, nice streets, great fish and chips, and local beer. Jet museum. History. Stone. Only spent a day there though, so not as familiar with it as Penzance.
There may be some similarities in my favourite sort of seaside towns