...what is/are believed to be the worst floods...

[Tan Elaine]

Thailand is drawing in what are believed to be the worst floods in more than half a century.The north and central plains have now been submerged in water, in places as high as two metres.

Should 'is' be used instead of 'are'?

Thanks.

 

[5jj]

No. 'are' is good.

Should the third word be 'drowning'?

 

[Tan Elaine]

No. 'are' is good.

Should the third word be 'drowning'?

Yes, it should be. It's a typo. Good catch!

 

[bhaisahab]

If you want to use "is", you could change "floods" to "flooding".