As i know the Best choice is Gastrodon from Azurine islands, but it' unavailable for now(pass to Reborn is closed) so the next choice is to catch spheal/sealeo, floatzel or seel/dewgong a bit later in citrine /celestial mountain (don't exactly remember how it named), but if you have some problems with Rock types it will be better to pick solo water type without ice. Also you will need someone to use surf and dive, and also the next leader is fire. Maybe i missed something, but that's all i can recomend