As an owner of a similarly-aged (21 going on 22) horse, I’ve taken my standardbred mare on the beach many a time. Unfortunately not so much this year as I’ve been putting milage on my younger horse, but when my STB was fit and of good wind (riding several times a week, including 6-8 hr hacking) I would let her run as far as she felt like. She was fine regulating her own pace. She never really “liked” the waves on the lake but she was okay with the water itself once we were in or if we crossed runouts and into tidal pools.
Out in the lake:
http://www.facebook.com/photo.php?pid=2522675&l=96998ff3d9&id=515023528
The draft cross I had loved water from the first time we tried it. Good thing, since we went bareback body surfing that first day (grin)
http://www.facebook.com/photo.php?pid=3601279&l=6625359d60&id=515023528
She played around the first time back to the beach with her new owner a year after I sold her and I wasn’t buying it. I got up on her and out we marched into the waves. This photo above was that day. As you can see, she’s tramatized 
Now my TB mare, she’s another story. She bolted on me the first time on the beach (whoo hoo racetrack!), and I came off when she went left spooking from a run out and I went right, hitting the hard wet packed sand. She was good at w/t but when everyone started cantering she decided she was a racer again and shot after them. I had no brakes and just race type steering after I lost my stirrups when she launched into the gallop. Ouch. I steered her to the waterline to avoid the other riders but didn’t have enough leverage to pull her up. We rode another 2 hours after that, but no more galloping down the beach. Took her back to the beach on the same day as the draft cross, and she was better on the sands but bucky when the water hit her ankles. I got her to stand with her back legs getting wet and called it progress. It’s going to be baby steps with her :rolleyes: