tip, at the water's edge, bring to its knees
1 "This is really the year that mobile has tipped for consumers," says Alexis Rask, chief revenue officer for mobile shopping app start-up Shopkick, which gives users points for walking into partner retailers that can then be redeemed for gift cards and other prizes.

2 "There used to be a pretty important principle that originated in the Republican Party, I believe, that partisan politics should stop at the water's edge," said White House spokesman Josh Earnest. " And it's unfortunate that we did see a number of Republicans yesterday who criticized the president for a handshake at Nelson Mandela's funeral."

3 He was a guerrilla, the commander-in-chief of the ANC's armed wing. Umkhonto we Sizwe, which, as the " Spear of the Nation", was supposed- however implausibly-to lead an armed insurgency, organise an invasion by sea and bring the government to its knees.

What do these phrases mean in the sentences? Thanks

Question

tip, at the water's edge, bring to its knees
1 "This is really the year that mobile has tipped for consumers," says Alexis Rask, chief revenue officer for mobile shopping app start-up Shopkick, which gives users points for walking into partner retailers that can then be redeemed for gift cards and other prizes.

2 "There used to be a pretty important principle that originated in the Republican Party, I believe, that partisan politics should stop at the water's edge," said White House spokesman Josh Earnest. " And it's unfortunate that we did see a number of Republicans yesterday who criticized the president for a handshake at Nelson Mandela's funeral."

3 He was a guerrilla, the commander-in-chief of the ANC's armed wing. Umkhonto we Sizwe, which, as the " Spear of the Nation", was supposed- however implausibly-to lead an armed insurgency, organise an invasion by sea and bring the government to its knees.

What do these phrases mean in the sentences? Thanks

Answer

'tipped' relates to old-fashioned weighing scales. If you gradually add more to one side, the scales will eventually tip, and that side will move down. It means that there has been gradual change, leading up to a sudden shift.
'The waters edge' refers to a logical, or moral border. In this case, they are saying that it would have been rude for Obama not to shake Castro's hand.
'to ones knees' refers to someone who has been beaten, and ends up 'kneeling' (usually figuratively) begging for mercy from whoever has defeated them.

Answer

Even as a well-educated native speaker, I didn't understand #1 or #3.  I did understand #2 though. "partisan politics should stop at the water's edge" means that partisanship (republicans always agreeing with each other and disagreeing with democrats, no matter what the issue, and democrats doing the same), should only happen WITHIN the united states. Once you cross an ocean, American politicians (or more specifically  republicans, in this case), should support each other and the nation, rather than republicans criticizing Obama basically because he is a democrat and they want him to look bad.