Tapirs love to swim, a large pool is a must. when i was in Osnabruck zoo (Germany) i saw the whole group swim togther.
about the interaction with the siamangs...i didn't see it, the siamangs were on the tops and the tapir just swam around the island.
I was looking at the map, and the tapirs have land area on the rear shore. Judging by the vegetation on the siamang island, I would guess the tapirs do not have access.