Yes, Ontario Has Freshwater "Seashores" on the Great Lakes

Ontario's southern boundary traces four Great Lakes—massive freshwater bodies that function like inland seas:

• Lake Erie – Warmest, sandiest: Long Point, Port Stanley, Erieau, Crystal Beach
• Lake Huron – Longest coastline: Sauble Beach (world's 2nd longest freshwater beach), Wasaga Beach (longest), Grand Bend, Ipperwash, Southampton
• Lake Ontario – Toronto Islands, Sandbanks Provincial Park (giant dunes), Presqu'ile, Cobourg
• Lake Superior – Rugged, cold: Agawa Bay, Pancake Bay, Katherine Cove

Features

• Sand dunes, warm shallows (Erie/Huron), clear water, no tides/sharks
• Provincial/national parks, campgrounds, cottages
• Swimming, sailing, kiteboarding, fishing, ice fishing

Not saltwater, but functionally seashores for recreation and tourism.