You'll get as many recommendations as there are NZ divers on here!!
Unless you specifically need to do day trips (e.g. non diving partner), then you get more bang for your buck with a liveaboard. Both OceanBlue and Pacific Hideaway are good options. OceanBlue is more likely to have space, PH is a larger boat and often booked out by local dive shops. Kev who skippers OceanBlue also cooks a pretty mean curry which is almost worth it on it's own.
They provide a dive guide, but like Tara said you don't need a guide in terms of finding your way around. Though a good guide will point out the cool stuff that you might otherwise miss. The real beauty of the PKs is in the small creatures on the walls - if you cruise past the wall more than a few feet away, you'll mostly just see a blaze of encrusting life but none of the detail.
My personal preference for day trips is Dive! Tutukaka for a whole bunch of reasons. Firstly, you can almost guarantee that they will be going out on that day. Some of the other charters require minimum numbers that they don't always get. This is particularly an issue as you are planning to dive midweek at the tail end of the summer season. Some of the other charters simply won't have the numbers to warrant them going out.
D!T have a policy of always going if they can, the only thing that ever stops them is the weather. They've also had a bit of a renaissance in the last few years, a previous criticism has been that they always go to the same spots and are a bit rigid in terms of how the day runs. I've not noticed that in the last few years and have found them to be very flexible and accommodating.
Tara's recommendation of Yukon is also a good one, Noel is a crusty old bu&&er who adds flavour and character to a day out!
If you want to make the most of your days be squeezing in three dives, then you probably want to look at the fast boats - Bluzone, Poor Knights Divers and Adrenalin Charters have boats that will get you out to the islands in 30-40 minutes (as opposed to 50-60 minutes for others). I've never dived with Adrenalin, so can't comment on them. Both of the others may struggle to get sufficient divers for a midweek trip, but their boats are certainly not going to be crowded. I'm not a huge fan of either of the boats - Bluzone's boat (Rumblefish) has no shelter and PKD's boat (Shadowfax) isn't really set up for a 3 tank day.
So, given it's midweek - my suggestion would be D!T.
Shame it's not the 20/21. I could go for a dive on the 20th and would happily come along and show you around, but I can't get time off on Thursdays and Fridays.
