Rain, low cloud over water its can be all the same colour in a heart beat.
So it happens.
the pilot continued to track along the planned coastal route towards a thunderstorm, probably encountering conditions such as low cloud, reduced visibility and turbulence, and as a result of one or more of those factors the aircraft descended and collided with water.
https://www.atsb.gov.au/publications...-2013-063.aspx
Safety message
Tracking visually via a coastal route in marginal weather conditions can be advantageous in terms of ease of navigation and absence of elevated terrain, but can also increase the risk of spatial disorientation in the context of drastically reduced visibility exacerbated by a lack of surface definition when over water
And
One of the fatal accidents occurred on 17 November 2007 when the pilot of a Cessna 337,
registered VH-CHU, was tracking along the coast from Moorabbin, Victoria to Merimbula, New
South Wales. In its investigation into this occurrence, the ATSB found that while manoeuvring
over water at low level in conditions of reduced visibility, the pilot probably became spatially
disorientated and inadvertently descended into the water. The investigation report on this
occurrence, number AO-2007-061, is available at
www.atsb.gov.au.