This is partly a problem with the episode ordering of FOX, and partly untold.
"Summer of Love" was supposed to be episode two. It was to transition them from the "timer can open portals whenever, only it's super dangerous and might short out at any time if not given enough rest" model of the pilot to "the weakened timer can open portals at one specific time at each world, and if they miss it they're stuck there for 29 years" model used in the rest of the series. However, because FOX executives decided to alter the episode ordering (putting what it saw as the most exciting or easy-to-advertise episodes early on when the show was seeking an initial audience... see this question for more details.) when "Summer of Love" finally aired they'd already been using the "29 years" rule in several previous episodes, it got very confusing (and the scene explaining why the timer has a 29 year rule was cut), leading to constant fan questions of "why don't they just advance the timer, even if it's dangerous" because they did at the start of this episode.
The other major problem with the episode ordering changing was with the beginning and ending of episodes that were meant to be connected, most notably with the situation your question describes.
When the episode ended, they did indeed face a Tsunami with too long on the timer. This was meant to transition directly into episode 3, which was intended to be "The Prince of Wails." Unfortunately, "The Prince of Wails" aired BEFORE "Summer of Love" and so this got especially confused.
"The Prince of Wails" opens with them on top of a high tower surrounded by water (and with a shark nearby). When the episodes are played out of order, it looks simply like one episode they open with a difficult situation on a weird world (as they often opened episodes with, without explanation), and in an unrelated episode they end the episode facing an impossible situation that never gets resolved... and in these two cases both happen to involve water. But you can see other clues to the proper order... most notably, in "The Prince of Wails" they are STILL in their 60s style clothes from the episode "Summer of Love."
We never actually see them 'escape from the tsunami' between episodes, but it's assumed that they somehow manage to climb the tower fast enough (or perhaps got into a working elevator or something and climbed from there). Compounding the problem with the episode scheduling, which the people producing the series had no control of, there was writing and staging consistency problems (such as it being dark when they had '45 minutes' to seeming like well into the morning when they actually had to slide, or whether tsunamis really would work that way, or if escape was at all feasible), which pretty much was par for the course of Sliders as a whole.