Really nice photo Rob, Thanks for sharing! I have been looking at Google maps and trying to work out where you took this shot from? (I’m South UK too, so looking at a drone trip one weekend!). Flying over sea still makes part of me pucker though to be honest! 
I’m standing amongst the rock groynes that form part of the sea defences.
Flying over the sea presents a few challenges. One potential problem is flying too close to the surface - obstacle avoidance and VPS gets confused by water (the same is true for inland waters). Having said that the surface tends to be relatively flat.
Spray from waves hitting rocks is a potential problem - again if you stay 5 metres or so above the surface you should be safe… I haven’t found any problems with seabirds - yet. No doubt others will have different views 
Be sure of your equipment. Batteries should be in good shape - I was testing some recently purchased Inspire batteries and one decided to fail when full power was requested. Failure over land may have left me with bits to glue back together, failure over the sea meant that the aircraft and electrics, when recovered, were no use at all.
Basically flying over the sea is no big hassle, it’s a problem that exists more in the mind than in reality.