Many marine engines simply don't have a drain plug, or it is too inaccessible even for a hose connection. Draining the oil from these engines requires a pump to suck it out through the dipstick tube.
On some engines the dipstick tube is threaded. The supplied oil-change pump screws directly to the dipstick tube and the oil is removed with a few pulls on the handle. This works fine where there is good access. Otherwise, you can mount the pump to a convenient bulkhead and connect it to the dipstick with a hose with threaded fittings-essentially a very short garden hose. Mount a short length of PVC pipe fitted with a male hose adapter next to the pump to provide a place to "park" the hose when not in use.