I have rental property, both long and short term, in Port Isabel. It is a different market than Port Aransas. My parents owned a condo in Aransas for several years growing up and I always liked it up there. Here is what I would say:
The only way you will truly come out alright on the short term rentals is if you do it yourself and do not have to a management/maint. fee. Those services are going to take between 30-40% of the rental off the top and then along the way they are going to charge you to replace the toaster, fix the toilet, buy 3 new spoons, etc.
In a really good property you might break even off the yearly costs of ownership, key word might. But the Aransas scene is big time slanted to the summer time and now to a lesser extent Spring Break. I think the winter rentals are picking up around there, but it is nowhere near what it is on SPI/PI.
I do real well with my winter Texan rentals as I have private place and I also get some really good business from Mexico during the summer. I mainly do weekly rentals, so I craft my rental calendar in a way to keep some of the summer open for use.
The thing that always made me leery about the rentals in that area is that there is very little market for long term rentals. Thus, if you just wanted rent the place on a yearly basis it would be really hard. Doing short terms and not handling them yourself or via long distance is hard, thus you have to go with the rental pools or management companies. In thinking about it, it might be OK if you can do the summer rentals via the management company and then get a good 3-4 month winter rental directly.
Based on my limited and questionable knowledge, I would not look for much appreciation of condo's anywhere on the Texas Gulf Coast for a long time.
I wish you good luck and I hope it all works out well.