As regards Eurostar, I've never really understood why there is a UK passport check by UK Border Agency staff, whilst this doesn't happen at airports ...
The legal framework around the tunnel is *unique*. Although aviation is a bunch of international treaties they do not impinge on frontier controls as far as I can think.
The tunnel and services through it can only work within the bilateral treaty with France (Canterbury) which explicily states that frontier controls are agreed in a protocol, Sangatte in this case. If either HMG or France want to change anything it needs a new agreement, which has happened several times including the juxtaposed control agreements.
Both governments have the same security mindset, so I do not know why they have not imposed advance passenger information on the tunnel, although the shuttle operations would fight against it unless imposed on the ferries too.
Worth adding, my limited understanding of maritime law is that anything can operate anywhere as long as the vessel is safe and safe to carry what it is carrying. If that is right, it would take a new bilateral with France or whoever to force advance passenger information to be collected whereas with aviation HMG can require it as a condition of carrying passengers here. Of course the Le Touquet agreements enabled juxtaposed controls on all modes.
Does anyone know more about a) the legal side of this and b) whether the governments have tried to force API on cross-channel?