Remember that Wordsworth was a Romantic, which means that all of his poetry concern man and Nature, and in particular the way that Nature helps us find solace for our soul and also can instruct and teach us through our contemplation of its beauties. So much of Wordsworth's poetry is concerned with these themes, and this one is certainly no exception.
The sonnet has a couple of strong examples of figurative language. The calm evening is compared to a quiet nun who is so caught up in prayer that she is breathless. This image reinforces the idea of the calmness from the first line. The image of heaven resting on the sea is also very peaceful and suggests that fine line in the horizen where sky ends and sea begins. This too is a calm image. The next lines break the calm images and contrast them with the "Being" meaning the ocean being awake and alive -- the waters of the ocean may be calm, but they are always alive and moving -- everlastingly as the poem states.
This piece is one of William Wordsworth's. It is in sonnet form, and quite frankly it is a simple as its title. Okay, there is a little more to it than that. But it does stay within (as is most of Wordsworth's style) a pleasing and tranquil mood. The speaker references nature's beauty and exercises several hints of a spiritual element.
He refers to a young girl that many believe might have been his illegitimate daughter. This poem might have referenced a time they spent together. The mood continues to feel gentle to the very end.