http://imulead.com/tolimared/submarine/

The boat on its bouy
In its final state the boat has a tower which keeps the waves from swapping into the hatch. The hull now with ballast is below the waterlevel only the tower comes out of the water. A pair of snorkels in front and aft part keep air circulating trough the hull on anchor place which creates a fine dry ambient without condensation in the hull.
The boat is anchored on a bouy in Steinbach in a field of small sailing yachts. No infrastructure of maintainance is needed. The hull needs no sandblasting and repainting during decades as any other marine concrete application. The only maintainence is scratching the alge and marine live from the hull with a rope passed below the hull and cleaning of the viewports with a brush on a stick. The only piece of infrastructure is the bouy and the red plastic boat for transfer to the anchorplace.
In ten years i have not seen any alteration of the concrete that could make a repair or painting of the hull necessary, so i would calculate the livespan for a hull like this with over 120 years. This is similar to what engineers calculate for other underwater concrete strucktures like drilling platfroms piplines and the canal tunnel between france and britain.