The first stanza describes a ship on still water. With no wind, the sails "from Heaven" are motionless. Inchcape Rock is a section of land or a reef in Scotland that sticks out only inches above the water during low tide and is completely submerged during high tide. So, a bell was attached to the rock by Abbot of Aberbrothok so sailors would be made aware of the submerged reef. This is explained in the third and fourth stanzas. However, in the second stanza, the speaker notes that the waves were so weak that the bell was not sounding.
On this particular day (fifth stanza) the weather is pleasant. In the sixth stanza, Ralph the Rover (possibly a pirate) spies the buoy of the bell. In the next stanza, the speaker notes his joy but adds that Ralph's intent is actually full of wickedness. In the eighth stanza, he commands his men to put him on a row boat so that he might approach the bell. In the ninth stanza, he approaches the bell and cuts it from its float. The bell sinks and he claims that the next sailors who approach will not bless the Abbot. (Evidently, Ralph is jealous that so many sailors bless the Abbot rather than himself.)
The eleventh stanza tells of how Ralph sails away, accumulates riches, and sails back toward Scotland. In the next stanza, Ralph's ship sails into a fog and there is no visibility. In the following stanza, it is night and Ralph says to wait for the light of the moon in order to see. In the following stanza (14th) one of Ralph's sailors can hear waves crashing and he wishes he would be able to hear the Inchcape Bell.
In the fifteenth stanza, their ship strikes Inchcape Rock. In the following stanza, Ralph curses himself as his ship begins to sink. He realizes his wicked decision to remove the bell has led to his own disaster and presumably his death. In the final stanza, Ralph hears or believes he hears Death's bell, comparable to a funeral bell: a fitting end considering he cut the bell that saved sailors like himself.