Just before you can actually attempt to cross, the pony who separated from your group earlier appears to be heading your way, clutching a vividly pink umbrella in her hooves. Before either you or Rarity can actually attempt to warn the white earth pony of the perilous crossing, she steps onto one of the bridges, and through a ridiculous series of what can only be described as cartoonish chain reactions, causes the bridge to break and sink beneath the waves - followed by every other mini-island and bridge connecting them. At the same time.
While she profusely apologises to the pair of you, and Rarity begins to toss up trying to comfort the poor dear while also resisting the urge to yell at her for her blatant carelessness, you take stock of your new problem. The stone slab with the pillar of light is still intact, but now it seems to be located directly at the centre of a perfectly square moat of seawater, stuck on a very small island. By your estimation (well to be clear, it's a very exact estimate, puzzles like this engage the brain in many ways) the distance between the three of you and the slab is three metres worth of sea water. You could probably make a running start and jump to the other side, but you suspect you probably couldn't make the crossing back, and your white earth pony baggage has said she can't swim.
Fortunately, not all is lost, for at that exact moment, Rarity spots two incredibly splintered trees, most likely exposed to the elements for too long, and proposes that they be fashioned into two planks of wood, and that the three of you use them to build a bridge to the other side. Having nothing better to suggest, you watch Rarity uproot the trees with impressive force (without magic, impressive what a bit of greed can motivate a pony to do) and whittle down the ruined trees into two planks, armed with only her filly Scout pen-knife and the aid of a time-skip found only in literature works, until finally she has two identical planks of wood.
Slightly less fortunately, when the time comes to actually test the planks, you discover that they are only 2.9 metres long: less than the 3 metre distance to the slab, and no matter how much you stretch, you can't do better than that.
Now what? You have absolutely nothing to tie the planks together securely: no rope, tape, or its big older brother GAFFA tape, and none of you fancy your chances at swimming for it. So how can you reach the stone slab that will take you to the next part of the treasure hunt, without getting your hooves wet or falling in the water?