1) The two aspects being identified are the characteristics of the dog and the sea. James Reeves is personifying the sea as if it were a dog.
2) James Reeves is mentioning the vastness of the sea by calling it giant. By stating the sea as grey he means that it’s opaque and to give the image that the sea is strong and frightening.
3) Both the dog and the sea roll on the beach throughout the day. The dog runs and rolls along the sandy beach and the waves from the sea are the ones swashing through the sand and rolling in and out constantly. They both also gnaw the dog will nibble anything it encounters and the sea by « gnaw » means nibbles on the sand by taking it off the beaches. The Dog growls and the sea makes it’s « growl » by the swooshing of the waves and their movement. The dog plays along the wet cliffs while the waves crash against the cliffs and erode them. Both the dog and the sea lay along beaches night and day. From time to time the sea calms such as in May or June as does a dog from the heat.
4) As the eats up and swallows the stones and shells and everything related the dog is on a hunt for bones. Therefore the equivalent of stones for the sea is the bones for the dog. This is one of the reasons that James Reeves write “bones” 4 times repetitively.
5) While the dog is “shaking his wet sides over the cliffs” the sea is crashing against the sides of the cliff roughly and eroding away the rock.
6) The poet is describing the clashing of the waves and its slow then speed up movements that never stop.
7) This “dog picture” tells us that the dog is calm therefore so is the sea.
8) The repetition of “s” sounds is a sibilant, which represents the swooshing movement of the sea. This gives us the image that the sea is calm and slowly