Some 137,000 made the perilous journey -- and the large majority of them were fleeing from war, conflict or persecution, the UNHCR report said.
This makes them refugees rather than economic migrants seeking greater opportunities in Europe
One-third of the men, women and children who arrived by sea in Italy or Greece were from war-torn Syria
, said the UNHCR. The second-most common countries of origin were Afghanistan and Eritrea, whose nationals also usually qualify for refugee status.
"As Europe debates the best way to deal with the rising crisis on the Mediterranean, we must be clear: most of the people arriving by sea in Europe are refugees, seeking protection from war and persecution," said António Guterres, the U.N. high commissioner for refugees.
The 137,000 people who made the journey represent an 83% increase on the same period last year, when 75,000 made the crossing, the report said.
And this could be just the beginning.
The second half of the year traditionally sees a big rise in crossings, as those boarding boats that are often barely seaworthy seek to take advantage of calmer summer weather. The lack of legal routes into Europe gives them little choice but to risk their lives, said the UNHCR.
One sign of hope is that after an unprecedented number of migrants died at sea in April of this year -- with 1,308 refugees and migrants reported drowned or missing -- the death toll dropped dramatically in May and June, as international rescue operations were stepped up, the report said.
In May, that number fell to 68, a quarter of the total lives lost in May 2014. In June, it fell still further, to 12 deaths, compared with 305 a year earlier.
"The decline in people drowning over the past two months is encouraging; a sign that with the right policy, backed by an effective operational response, it is possible to save more lives at sea," said Guterres.
"Nonetheless, we must stay vigilant. For the thousands of refugees and migrants who continue to cross the Mediterranean every week, the risks remain very real."
Guterres: Europe has clear responsibility
Most of those making the journey arrive in Italy
The latter country, struggling with its own economic crisis
, has limited infrastructure to handle the flood of refugees, said the UNHCR. As a result, many continue onward across Macedonia and Serbia into Hungary and face an additional threat from unscrupulous people, such as traffickers and criminal gangs.
"More than 85 per cent of those arriving in Greece are from countries experiencing war and conflict, principally Syria, Afghanistan, Iraq
," said the report. "From Greece, most move onwards across the Balkans to western and northern Europe. Italy remains the primary destination for Eritreans, Somalis and other people from sub-Saharan Africa."
The UNHCR has appealed for Europe to step up and help more of those in desperate need, in line with its core humanitarian principles.
It points out that European nations took in a relatively small share of the total number of refugees last year, with Turkey, followed by Pakistan
, Ethiopia and Jordan, taking in the most.
"Europe has a clear responsibility to help those seeking protection from war and persecution," said Guterres. "To deny that responsibility is to threaten the very building blocks of the humanitarian system Europe worked so hard to build."