Boat radical - 艘 is the measure word for ships and large vessels (一艘船 'one ship'). Pictograph of a small boat seen from the side. Same radical drives 船 ship, 舱 cabin, 航 voyage, 般 sort/kind.
Phonetic - 叟 sǒu → 艘 sōu (tone shift only). 叟 originally pictured an elder holding a torch in a house; here it lends pure sound. Same phonetic appears in 嗖 whoosh, 馊 spoiled, 飕 rustling-wind - all sōu/sǒu family.
