boat slide
/ˈbəʊt slʌɪd/noun
A slide or ramp up or down which a boat may be moved; specifically (a) one used for launching or beaching a boat; (b) an incline, typically fitted with rollers, over which a small boat travelling on a canal may be drawn, instead of passing through a lock.
Origin
Mid 19th century (in an earlier sense). From boat + slide.