Baston,
Neither ac nor ship radars would have given a complete picture and ATCers were out of the loop as both pilots had called a positive 'visual' while at LL over sea.
Quite why they remained on a collision course, no-one will ever know but the BoI highlighted the confusing and disorientating lighting configuration produced by an ac with its fwd HISL 'off', the tail HISL 'on' (but dimmer and partly occluded by sets of rotors) and the nav lights flashing bright. If both crews were 'visual' and yet took evasive action only at the last minute then it is highly likely that they may have been disorientated and confused as to the relative movement, closure and speed of eachother.