No, your sump will be just fine.
What you have to find out is where the crack is. It is most likely on a seam, which is good because you can fix it with locally available cements. It shouldn't be too hard to locate, there should be significant salt crusting around the cracked area.
Check your bulkhead fittings first if any are submerged. They are the most likely point of leaks.
Here are your two fixes:
If your crack is in the middle of the plastic somehwere:
Order the smallest possible can of IPS Weld-On #3 from Tap Plastics, and an applicator with a 25 gauge needle. US Plastics sells one called a "plasticator." Keep using towels until it comes.
When these items arrive, Drain the sump and dry thorougly with a hair dryer, leaf blower, etc... something to make sure it's bone dry. Leaving it in the hot sun for an hour will also work. Fill the applicator with a small amount of the weld-on. Apply a small "trickle" along the crack. The crack will wick the cement in and dry within 30 minutes, and you'll be good to go. The crack will be literally fused shut, so the repair should last the life of the sump.
If your crack is on a seam
Drain the sump as explained above, and use ordinary Testes Model Cement (or whatever brand) to sort of "caulk" the crack from the inside of the sump. Let dry for several hours and you're done.
Both fixes will work in both circumstances, but the model cement tends to be uglier, so I don't recommend it for a crack in the middle of the acrylic. Conversely, I don't recommend going to the trouble of ordering weld-on unless you already have it for a seam crack as model cement isn't really visible on a seam and it's locally available.
Other sealants that will work are: Marine Goop, Duco Cement, 100% Silicon (though not well, it doesn't bond very well to acrylic). All of these products need to dry overnight though.
