Finally, someone is doing something in the poem! We learn that the speaker is sailing. He reaches ("gains") the "cove" (a kind of recess or sheltered space on the coast of an ocean).
The descriptions in lines 1-4 refer to the scene the speaker observes while sailing.
"Quench its speed" is strange, in part because we don't know what "its" refers to. It seems likely that "its" refers to the boat the speaker is sailing.
"Quench" means to extinguish or stop (like quenching your thirst by drinking Gatorade), so "quench its speed" means to "stop" the boat on the shore, "i[n] the slushy sand.