Broadly speaking, the answer's in the title: it's about a flood, witnessed by the poet's speaker as he stands on 'Lolham Briggs', a viaduct bridge in Clare's native Northamptonshire, crossing the flood plain of the River Welland. Clare describes the power of the flood in three stanzas, each of them a sonnet, the tight organisation of the sonnet form providing a tension with the awesome and uncontainable power of the flood itself.
The lack of formal syntax, common in Clare, means that the reader has to supply the 'sense units' to infer meanings, but the ambiguity is often fruitful. In the opening lines, for example, 'in wild and lonely mood' could refer either forward to the speaker, or backwards to the setting on Lolham Briggs.
In the first stanza, the poet watches as the flood beats against the arches of the bridge below. Both the bridge and the flood itself are lightly personified: the bridge 'breasted raving waves', as it withstands the terrifying power with a 'shudder'. Description dominates over any implied meaning until the last line of the stanza, where Clare draws a comparison between the movement of the water (at one moment distinct eddies, at the next 'engulphed' in the general motion), and human life, inexorably swallowed up by death.
The syntactical ambiguity continues in the second stanza, as the 'wrecky stains' could apply either to the eddies of earth-stained flood water, or to death itself. This verse continues the description with a more minute focus, observing things like straws and feathers floating, turning and being borne away on the stream, 'seeming as faireys whisked [them] from the view.' By the end of the stanza, more substantial things ripped down by the storm - 'bushes, fence demolished rails' - are seen 'loaded with weeds', like 'water monsters'.
In the third stanza, as at the end of the second, the rolling motion of the flood water is captured through the mid-line breaks: 'Waves trough - rebound - and fury boil again'; then Clare visualises the relentless movement of the water on towards the ocean - 'like trouble wandering to eternity'. Unlike most Romantic poets, Clare does not habitually use nature as a reflection of spriritual states, or of the mind and heart of the poet - his presence in the poem is as perceiver only. But that perception is minutely vivid and evocative: from the intriguingly onomatopoeic 'huzzing sea', to compound words like 'waterstrife' and the ineffably mysterious and alliterative 'strange birds like snow spots', which 'hang where the wild duck hurried past and fled'.