Technique for holding a horizontal minifig in place (swimmer)
Mmmh... If you base doesn't have to be all solid...
Recess some piece in there in which you can insert the arm-side of a mini-fig hand.
Grasp your swimmer by the toes on one of its feet, with that hand.
That will give you a horizontal/slightly upwards position on the mini-fig and you can further increase the angle by adjusting the mini-figs leg.