Yep, 25 per day with catching days - Monday, Wednesday, Friday, and Saturday. You need to have a freshwater license. And DEM does frequent that run...
Best places to get them are tough to get to. Some people actually walk the bease edge of the falls but that is very slipper and very dangerous. I got knocked off from there last year and was sore for a week... Not reccomeded... Good spots to get them are in the pool that forms near the ladder (you cannot legally extract fish from the ladder) the fish can often be found falling off the falls just to the right of the ladder but wading is slippery and there are two REALLY deep holes to watch out for... The other side by the little park/Architectural Firm can be the better side at times where you can fill up fast 10 feet from the right side of the falls... But the best thing (and sometimes most frustrating) is to watch where they drop over the falls in bunches and scoop them up in your net... They also pool a little down stream but not as well as up at the falls...
That place has several factors that can help you break your neck - just be carefull...
They other nice thing about that run is you load up Friday night and fish them when the bucket brigade is lined up at the ditch waiting for there 5 fish. The other nice thing is that the Watertown Run tends to hold herring much longer (a few weeks) than Wareham, Middleboro, and certainly - the canal run.
The only down side? Some people are a little indignant that your fishing smelly Charels River fish in Cape waters
Specialist - here's a big map to the run and a little map:
http://www.mapblast.com/myblast/map.mb?&MA=2&grp=m&DU=MI&W=600&H=400&IC=42.36577%3 A-71.193766%3A650%3Aherring%20run%20&noPrefs=1&CMD=L FILL&CT=42.359262:-71.167129:50000&GAD3=Watertown%2c+MA&GAD4=USA&AD3= watertown%2c+ma&AD4=USA&Zip=02472
and the detailed map:
http://www.mapblast.com/myblast/map.mb?&MA=2&grp=m&DU=MI&W=600&H=480&IC=42.365292% 3A-71.190028%3A650%3Aherring%20run%20&noPrefs=1&CMD=L FILL&CT=42.365102:-71.191747:10000&GAD3=Watertown%2c+MA&GAD4=USA&AD3= watertown%2c+ma&AD4=USA&Zip=02458
If you stay on the Newton side of the river, you are on the fish ladder side. If you cross Galen St into Watertown, you're on the park side... Sometimes I've gotten what I neded in a half hour. Sometimes I had to work both sides of the run...