We always used polyurethane varnish on bogwood, but you need to give it probably 4 coats to seal it, it does dry to a very hard durable finish so I wouldn't think you'd have trouble with fish eating it.
The big problem is bog wood sinks when it becomes water logged, so if you seal it you keep the water out, so it remains bouyant and you need a heck of a lot of weight to make it sink.