You want a speaker that survives pool splashes and beach sand without dying after one season. Buy a rugged, high-quality model once so you aren't replacing it every year.
Tech brands usually refresh their lineups in the spring. Wait until after the summer holidays to buy, when retailers drop prices on the previous year's stock to make room for new inventory.
Remember that the 0% window is a deadline, not free money. If you have a balance left when the 18 months are up, the card will start charging interest on whatever remains.