First I would recommend that you get a copy of Campbell's Guide to Cruising Texas
http://www.waltereden.com/CGCT.html and
charts of the areas you plan to cruise. I would also suggest that you check out USPS’ Boating Guide to America
http://www.boatingguidetoamerica.com/.
If you’re looking to put in at Clear Lake, you could spend a night at Red Fish Island and then on to Galveston. BTW, the Houston ship Channel is vey busy with tankers and Tows. While you are east of the Galveston Causeway Bridge besides monitoring CH16, you need to talk to the Tows on CH13. Once you are west of the bridge, the Tows are on CH16. The intersection of the Houston Ship Channel the ICW and the Texas City Ship Channel is one of the busiest in the US, so keep a good look out.
If you are planning to go offshore, be aware that for the 300 miles along the Texas Coast, there are only 6 entrances from the Gulf – Sabine River, Galveston, Freeport, Port O’Conner, Port Aransas, Port Mansfield, and Brownsville. Many of the river entrances are either shallow of very prone to shoaling.
Galveston to Freeport can be travelled either offshore approximately 46nm (it’s 5nm to go out the Galveston Jetties which are taking you east) or the ICW which is about 37nm. Either way it’s a good days motor or sail. I’m guessing that you are looking to do this during the summer so you will have plenty of daylight. When we had our C25 we did both and it’s not a problem with the right weather.
Freeport to Port O’Conner is 62nm offshore and in a C25 it would be difficult to do during daylight and I definitely do not recommend doing the Matagorda channel at night. The current can be +3kts and depending on the wind you can have 6ft breaking seas. I’ve done it in a 40ft sailboat and don’t want to do it again. My suggestion would be to go the ICW – Freeport to Matagorda, Matagorda to Port O’Conner.
Check your tides and wind when entering or heading out of any of the inlets along the Texas coast. With an outgoing current of 2-3 kts and a south wind of +15kts, some pretty rough seas can be experienced. And if you have not had any experience offshore do not try to come in after sunset. It’s a whole different world doing this in the dark. Even being familiar with the area, the background
lights on shore make it very difficult to see the NAVAIDS.
You also don’t want to be travelling or anchoring in the ICW in the dark. Along the ICW, you will see many NAVAIDS taken out by the Tows.
Communicating with the Tows is a different experience and you can find an example at
http://www.tmca.nu/links.and.features/vhf.htm.
We did it when we had our C25 and had a great time, but it takes planning.