User blog comment:BannedLagiacrus/Monster Appreciation Week: Brachydios(4th Gen)/@comment-25231613-20150907120737/@comment-4640052-20150911211802

It is not likely that it would swim, but in case it does, it might as well survive it. It may be a dangerous ordeal that only the strongest Brachydios are able to survive ... but if this was a common behavior the Brachydios would have developed a resistance to water, so it is not likely to be the case.

The idea of having the Brachydios dig under the sea, or simply travelling through undersea caves in order to reach the Ingle Isle makes more sense than having them swim there.

I think there could be another way for it to get there. The island could have a temporary connection to the continent, maybe due to the sea tides, and only a few Brachydios are strong enough to get there before the tides change and drown them. Also, given that the volcanic regions show a constant pattern of movements, even tides on the lava flow, the volcanic activity itself could generate a temporary bridge to the island.