Border checks / passport ID down the Irish Sea have already been turned down by the unionist parties. I assume you're a unionist? If so I wonder how the general unionist public feel about their party leaders saying No (as is ever always the case!).
On the second point - Theresa May and the Tories have already ruled out a common trade and travel area with the EU. I agree it makes perfect sense but it's off the table unfortunately thanks to the current UK government.
Massive difference between EU travel and Irish travel. Irish people can move and live and work freely in Britain, and vice versa, under older legislation. Other EU citizens won't. They could get in through Ireland yes, but would not be entitled to work. That would definitely be a disincentive. I don't think either PM has ruled out a common travel area, the EU are just terrified that would mean no tariffs for them and are trying to dictate.
To be honest I haven't been on a boat in about 15 years so I couldn't be less arsed whether there is a sea border. If I need a passport and they check me at the border so be it. Its a minor inconvenience and not worth getting wound up about. Some unionists get way too wound up on symbolism. I'm a pragmatist myself. I would say yes I'm a unionist but I don't like a fair chunk of unionist politicians. Way too much time spent on meaningless symbolic gestures in this country, but no need to get into all our problems and history, or I'll be here all night...
Customs does however seem totally unnecessary. I don't know any other country with internal customs, it would be very odd. And I'd rather help businesses not waste their time with red tape.