All bodies of water in Texas are open to seaplane use, with the exception of Lake Grapevine which is under the section B veil of DFW Airport and a few others that are on military reservations. There are also a few lakes that are restricted to 5 hp or less and are closed to seaplanes, such as downtown Lake Austin and White Rock Lake in Dallas.
Texas has some outstanding lakes such as Lake Texoma spanning the Oklahoma and Texas boarders. Cedar Mills Resort and Marina on the Texas shore is an outstanding location and they sponsor they sponsor an October Seaplane Fly-In each year. The only seaplane base in Texas is David Wayne Hooks Airport, whish has a 2,500ft water lane for landing and takeoffs on the east side of the airport.
Many of the lakes are US Army Corp Lakes and have specific restrictions on landing and taking off near shores. Please reference the SPA Water Landing Directory or US Army Corp website for specific restrictions about each lake.