‘I started Early - Took my Dog’ recounts the speaker (possibly Dickinson herself) walking a dog at the seaside. She sees ‘Mermaids’, possibly seals or sea elephants, emerging from the sea. There are also warships, beside which the speaker realises her smallness, the size of ‘a Mouse’.
The wave then starts whipping on her. The gradual invasion is sexually charged.
But no Man moved Me - till the Tide
Went past my simple Shoe -
And past my Apron - and my Belt
And past my Boddice - too -
The bodily violation makes the sea all the more erotic. The wave relishes the speaker bit by bit, first the feet, then the lower and upper body. The wave is then personified.
And He - He followed - close behind -
I felt His Silver Heel
Opon my Ancle - Then My Shoes
Would overflow with Pearl -
The pronoun ‘He’ has received much Freudian and biographical attention on Dickinson’s romance. ‘He’ is neither God (as in the poetess's ‘God is indeed a jealous God’) nor convincingly presented as the poetess’s lover. The personification of the wave can be erotic without being necessarily biographical and self-revealing.
Until We met the Solid Town -
No One He seemed to know -
And bowing - with a Mighty look -
At me - The Sea withdrew -
If the first impression of the sea is a sexual assaulter, then at the last stanza it is given an abrupt metaphorical turn to be courteous. It ‘bow[s]’ to the speaker whom it deems ‘Mighty’.
I came upon an interpretation that points us to Orion and his dog (a constellation) as the figures in the poem. I found it illuminating as a former student of Maths. You may want to have a look at that. For the interpretation on the heavenly stars, a user identified as Djehuty has great original insights (See https://www.online-literature.com/forums/showthread.php?17963-quot-I-started-Early-Took-my-Dog-quot).