The most unlikely starter last season came in the form of seventh-round pick Cody Grimm, who stepped in at free safety when Tanard Jackson was suspended indefinitely. The rookie turned out to be a very good player, despite a conspicuous mistake in coverage in his first game. He proved to be solid in coverage, but was most valuable in the run game. He did a good job filling alleys, and boosted the Bucs' run game. In addition, he's one of the best tacklers on the field. Unfortunately his season was cut short because of a leg injury, but he seems to have fully healed and seems ready to get back on the field. He should be the starting free safety once the season begins.
At strong safety, the Bucs brought in Sean Jones to replace the disastrous experiment of Sabby Piscitelli. Sean Jones was a solid safety, but not much more than that. He surprisingly notched only one interception after having multiple-interception seasons with the Eagles and Browns. Sean Jones hasn't done anything to lose the job and should be able to hang onto it. A solid but unspectacular player at strong safety is very useful, and there's no one on the roster who could challenge him.
That is, until Tanard Jackson returns. He's eligible for reinstatement starting with week 3 of the regular season, and is likely to return to the Bucs at that point. He is still under contract for one more season, and could step in immediately as the free safety if he's in football shape. The Bucs may want to think twice about that, though, as it's doubtful whether Jackson can be counted on in the long term. Despite that, Jackson was a terrific safety for the Bucs and has always been an instinctive player. It should not take him to long to get back on the field.
If Jackson does get back on the field, expect Cody Grimm and Sean Jones to fight over the starting strong safety spot.
Behind those three players the Bucs have a number of young backups. Vince Anderson and Larry Asante joined the team as rookies last season and should be fighting for the same roster spot. I don't know much about Anderson, but Asante managed to get on the field a little last season, even picking off Drew Brees in the final game. Asante's mostly a backup for the strong safety position who will do well in the run game.
Finally, the Bucs have fifth-round draft pick Ahmad Black. The Florida Gator was an instinctive safety in college, but will struggle to keep up in the NFL because of his speed and size. While the Bucs can adapt their coverage to his limitations, I think he will be most suited for special packages. Specifically, I can see him working as a defensive back working on tight ends and slot receivers in nickel packages. He has the instincts and short-area skills to do well in such a role, but I doubt he will ever turn into a starting safety.