Is it okay for preserved lemons to float?
While traditional recipes do not (apparently) use weights, other fermentation texts highly recommend submerging the material being fermented ("Fermented Vegetables", Christopher Shockey and Kirsten K. Shockey) and I had also observed possible mold problems when fermenting lemons myself. Therefore, standard advice would be to weigh the lemons down with something acid resistant (glass would be typical) as the high salt content and shape of the lemons makes them prone to rise above the brine.