I just talked to HR at MAT. You don't need any more float experience than it takes to get your SES. But you do need to have Twin Otter experience, even if it's only on land. According to him, they're not sending new hires through Twin Otter training at this time.

He did invite me to send my resume, and they'd keep it on file for if they change their requirements in the future. But without any Twin Otter experience, I'm not going just get an SES rating at this point.