Yep, as above. The Melinka, a converted landing craft, sails daily apart from the last Wednesday in the month, as I recall. You just show up and buy a ticket, they'll shoehorn you on last next to the ramp. It cost 6,900 pesos in '04, about UK£6.50. I remember it as being nearer four hours than two, but could be wrong.
The road from Porvenir to the Argentine border is fabulous, very good gravel, up and down and twisty and next to the sea on the first stretch. The only problem I encountered was nearly being taken out by guanacos.
Porvenir itself I thought was delightful, probably what Ushuaia was like before it became a tourist trap. There's a charming little museum and I stayed in the 'Triple X Video Club' hostel!
Be sure to spend at least one night in Rio Grande at Hotel Argentina, San Martin 64 (I think?), you won't be disappointed.