Hi,
Have used the Tasmanian ferries numerous times first in the 50's. the current ferry's were bought to give a high speed transit and replaced the old much bigger and more comfortable ferry, not a very practical move as its only 2 hours faster than the old one and uses hugely more fuel for less capacity. there was also a question about the price, but it was done by a government. Vehicle costs are subsidized by the Federal government and prices are set for normal fuel usage for a small car for the same distance.
its reasonably comfortable, used to have an Ala-carte restaurant and the waiter told us it has never been rough enough for a long stemmed wine glass to fall over, unfortunately this was closed for cost reasons so you only have a buffet now but food and wines are reasonable priced and quality is OK. cost of a ticket includes a recliner seat which is marginally more comfortable than an economy airline seat for the 13 hour crossing, you are not allowed to sleep in the very comfortable lounge chairs dotted all over the ship and its bars. there a shops and tourist offices on the ship, good wifi, movie theatre, sometimes live music in the main lounge. We strongly recommend a cabin [cost per person], all are ensuite, in which you can have a comfortable sleep and a shower etc.We have seen some people get seasick standing on a deck of a ship tied up to the jetty, but on the ship its more an engine rumble and slight vibration of the deck than actual roll, the ship is well stabilized. Note; its usually cheaper to fly but if touring consider it to be an expensive hotel room. hope this helps.