Like others have said, Dinner Plain to Jervis Bay is a long way for a day trip. It's a 9 hour drive from Lysterfield or Dinner Plain to Jervis Bay, and its over mountain ranges so altitude will be a consideration on the return trip.
Lysterfield to Portsea is definitely doable. If you do end up doing that, make sure you give yourself plenty of time if you're going to Dinner Plain after diving - Dinner Plain is at 1500m above sea level, and if you're driving via Mt Hotham, you'll reach an altitude of 1700m.
There's some excellent diving out of Portsea, including some great wall dives within the bay that range from 18m to 60m. There are also plenty of wrecks, with about 4 or 5 in the 26m-36m range that have regular charters out to them.
Bay City Scuba (
Bay City Scuba – Learn Experience Live) and Scubabo (
Scubabo Dive Victoria - Premier Boat Charters & Dive Courses!) are the two biggest charter operators.
Hiring kit may be logistically challenging. There are couple of dive shops in Rye, which is en route to Portsea, about 20mins away. ScubaDoctor (
The Scuba Doctor - Melbourne Dive Shop, Guided Dives, Air Fills, Service and Repairs) and Extreme Watersport (
www.extremewatersport.com.au/) come to mind. However you will unlikely be able to hire gear, get on the boat, and return the gear in the same day.
There are a couple of dive shops not too far from Lysterfield. Aquatics Adventures (
www.aquaticadventures.com.au) is the closest, but I don't know anything about them. I can recommend
Ocean Divers though (oceandivers.com.au) which is a little further.
Charter boats here in Melbourne are very much self catering. Sort out your own gear. Gear up in the carpark, get your gear down to the pier and make sure you get on the right boat at the right time. There are no dive guides, but the boat captains generally give good briefings. Virtually all dives are descent on a shot line and a free ascent on an SMB, so make sure you bring an SMB - shops don't generally hire them.
The weather has been attrocious here over the past week, with squawling winds and thunderstorms. Marine conditions have cancelled almost all diving other than some carefully chosen well protected pier dives. Forecast looks on the up though, so fingers crossed the swell drops and visibility improves by next week.
If that hasn't turned you off, there is definitely great diving to be had.
Depending on timing, you may want to do a boat dive in the morning, and then drop by Rye Pier on the way home for a shore dive. It's only about 5m under the pier, and gets out to about 10m if you can get some local knowledge to follow the path out to the octopus's garden.