Hi,
Garmin 310XT was never intended for lap swimming in a pool. It will NOT work, simple as that. It only supports open water swimming. The Garmin Swim, FR910 and Fenix 2 are the only devices that will allow you to log lap Swimming. These three devices are pretty accurate, although, they do miss or add an extra lap now and then. The technology is not perfect (and probably never be) and the watch is sometime confused by what's happening in the pool. If you apply the advice from those FAQ, you should get a pretty accurate count.
Why am I experiencing errors with my pool swimming distance?
Can traffic from other swimmers in my lane cause distance errors when Pool Swimming?
Even if there are sometimes mistakes, you can still get useful information from the watch. Most users understand the limitations and live with them. Some wish they could edit the data. Unfortunately, Garmin Connect does'nt allow that. But you can do it on my website for free and import the edited data back to connect.