Slitherine

Norway turn 2
UK Arty in boat in a harbor gets attacked by German tank and is insta killed as it scuttles
German tank attack UK infantry in boat next to arty, shoots and scores 0 hits.

Why?

While on the topic of Norway, why do arty and AT have such awful Anti ship stats?

You sort of answered your own question.

Because ground units have such horribly low attack values against naval targets (shooting rifles at a cruiser off shore?), if a naval target is camped on top of a victory hex port... ground units are powerless to take that victory hex.

So if a ship is in port and it is attacked by a ground unit, it is forced to retreat. Think of it has being ousted from port. If the ship cannot retreat because all naval hexes are blocked or occupied, than the crews sinks their own ships rather than let them fall into enemy hands (Cruiser in port self destructs before invading infantry storm and board the ship)