Hi,
We're planning a two-week trip starting April 6th. We'll start out in Bangkok (booking the Riverside Mariott through Priceline seems the way to go) and then want to head south for some beach time. Do we need to book ahead for the Phuket (probably Kata)/Krabi (probably Railay Beach) area? What about Khao Lak? What are the chance they'll still be running trips to Similan? I'd also love any advice on good dive sites for that time period.
Thanks for the help!

You should book your first few days without a moments daely, Thai New Year makes travel/hotels between 10th and 17th April very busy, although you are right on the tail end would be wise to get 16th-18th booked soon.
As far as Phuket etc are concerned it is not necessary to book ahead as plenty of accommodation however if you think about the logistics you would find yourself landing say at Phuket Airport and then you are at the mercy of the hotel booking counter or a taxi driver, you have no real control on what they will suggest to you, you will have no time to check locations or check reviews of what places are like, I think you gain nothing but actually end up with a headache(!). I really would not leave yourself at the mercy of an airport hotel desk, tout or cab driver.

I think for Phuket there are plenty here who can guide to towards some nice places (I lived there several years and visit often ) and others on the board have also been in the last few months.
Give us an idea of budget, Kata is always a good balance, I have suggested the Kata Thani to quite a few and thankfully ( breathes huge sigh of relief ) reports back have been very good. There are plenty of places for all budgets around Kata.

For Krabi everything is fine, busy area is Ao nang but to get to beaches you take local long-tail boats, so maybe staying around Railey would be best.

Personaly I would avoid Khao lak, although some resorts are now open there is a lot of work still going on, perhaps the one place that needs a few more months to be fully operational.

Dive trips 'are' operating from Phuket, you won't have any problem finding trips, on that side I 'would' leave until you get there, so you can see weather and also see what is on offer from the various operators. Similans should be ok but if you are after snorkeling then nearer trips from Phuket or Krabi are easy.

Others will tell you all about Priceline, but Marriott resort for BKK is a good idea, I have never fully grapsed the Priceline concept but my minimal knowledge of how it works is that you 'can get lucky' but not always.

Again, if you let us know your budget it's a good guide.

Flight BKK to Phuket/Krabi are cheap, to Phuket lots of options on carriers, Nok Air and Thai Air Asia are the budget carriers but Bangkok Airways and Thai Airways themselves have some very good deals at the moment.

If you want to 'down budget' at Kata Beach and are happy with a smaller a/con place the "Kata Garden Bungalows" are about 5 mins to the beach, this is 2 star a/con I would say, but has a pool, on the ridge between Kata and Karon Beaches, $30-35 twin inc breakfast/tax etc, they have a stay for 4 nights pay for 3 nights promotion until end April.

Railey East Sunrise Tropical Resort about $60-70 for April. You can road from Phuket in a few hours with a car+driver. You can also boat from Phuket Town but weatherwise I would go by road. You take a long-tail from Ao nang to railey so keep your baggage down.

Beachsiam.com and huahin.20m.com are in my opinion very good, lots of photos, very efficient, local too!

Thank you so much for all your help, JamesA! I'll book my Bangkok reservations today (we just got our tickets settled last night). We were thinking about the Khao Lak area because my husband's not really into a liveaboard trip, and a day trip to the Similans seemed more do-able there, but maybe not. We're more interested in dives than snoreling, though I do enjoy snorkeling as well. As long as I'm in the water! As far as budget, we want to stick to around $100/night (less is always nice, of course). I was thinking maybe the Coral Island Resort for a few nights, the Katathani Resort, and the Sand Sea in Railay Beach. Does anyone know what shape Khao Lak is in right now?

I misread your original thinking you were arriving 16th, if your 2 weeks are 6th to 20th then you need to settle accommodation for the period 12th to 20th without delay, and get those flights booked to! believe me, 12th to 20th April is absolute Peak travel time of the year within Thailand.
I was in Phuket a few weeks back, which is fine, even in Patong it is just more construction than normal and lots more of it, but Khao lak is very much a different scenario. You can check with dive operators by looking on Phuketgazette.net and also get some idea of Khao lak. I would give it a miss at the moment.
I really stress to get flights booked, Thai and others put on extra domestic flights but I know that they get booked up heavily. You shouldn't have any problem getting accommodation but if you want specific such as Kata Thani I would settle dates/plans as soon as you can.